Caramelized Onion Potato Thyme Slab Pie Recipe – Easy Puff Pastry Dinner

The first time I slid a golden, flaky Caramelized Onion, Potato, and Thyme Puff Pastry Slab Pie out of my oven, the kitchen smelled like pure comfort and a little bit of magic. The sweet onions, buttery potatoes, and earthy thyme all tucked into a crisp puff pastry—honestly, it stopped conversation at the table. You know that moment when everyone goes quiet because what’s on their plate is just that good? This is that kind of recipe.

I stumbled onto this delight when I was hunting for a dinner that felt fancy but didn’t require a culinary degree or a million ingredients. My love of savory pies comes from my grandma, who made everything from scratch, but let’s face it, sometimes store-bought puff pastry is the weeknight hero we all need. The first time I paired caramelized onions and potatoes with thyme in a pie, I knew I’d landed on something special. Since then, I’ve tested it every which way—different cheeses, vegan swaps, you name it. It always delivers.

This Caramelized Onion Potato Thyme Slab Pie is my go-to for feeding a hungry family, impressing brunch guests, or just making a Tuesday feel like a celebration. It’s not only easy (yes, even if you’re new to puff pastry), but it feels downright luxurious to eat. If you love classic French flavors—think savory tarts and comfort food—but want something approachable, you’ll adore this. I’ve leaned on this recipe for potlucks, cozy nights in, and even a few picnics. It’s that versatile.

Whether you’re looking for a hearty vegetarian dinner, a savory brunch centerpiece, or just a reason to eat more caramelized onions (no judgment), this slab pie is for you. After baking it more times than I can count, I promise it’s worth every flaky bite.

Why You’ll Love This Caramelized Onion Potato Thyme Slab Pie

• Quick & Easy: Thanks to ready-made puff pastry, this pie comes together in under an hour. Perfect for busy weeknights or when you need to whip up something impressive, fast.
• Simple Ingredients: No wild goose chases at the grocery store. Most of what you need is probably already in your fridge or pantry—onions, potatoes, cheese, thyme, and puff pastry.
• Perfect for Any Occasion: This slab pie shines at brunch, dazzles at dinner, and holds its own as a holiday side. It even works for picnics (just slice and pack!).
• Crowd-Pleaser: Kids love the crisp pastry and creamy potatoes, adults swoon over the sweet, savory onions and aromatic thyme. It rarely leaves leftovers.
• Unbelievably Delicious: The layers! The combination of slow-cooked onions, buttery potatoes, and flaky pastry is pure comfort. It’s one of those recipes where you close your eyes after the first bite.

Here’s what truly sets this Caramelized Onion Potato Thyme Slab Pie apart: The onions are cooked long and slow, coaxing out their sweetness and making the flavor so much deeper than your average pie. Slicing the potatoes thinly means they bake up creamy and tender, never undercooked. And fresh thyme? It adds a subtle, woodsy note that ties everything together beautifully.

I’ve made plenty of puff pastry tarts and pies, but this one always wins because it delivers so much flavor for so little effort. The slab shape gives you more crispy edges (the best part, in my opinion), and you don’t have to fuss with a pie pan. It’s comfort food that’s both familiar and a little bit special. If you’re looking for a recipe that feels like a hug, this is it.

What Ingredients You Will Need

This recipe uses simple, wholesome ingredients to deliver big flavor and that irresistible, flaky texture—no fancy ingredients required. Here’s what you’ll need for your Caramelized Onion Potato Thyme Slab Pie:

• Puff Pastry Sheets (2, thawed): Store-bought all-butter puff pastry is my top pick. Pepperidge Farm works in a pinch, but I love Dufour for extra flakiness.
• Yellow Onions (2 large, sliced thin): Sweet onions or Vidalia also work. The slow caramelization brings out natural sugars and depth.
• Yukon Gold Potatoes (2 medium, about 400g, peeled and sliced paper-thin): Their creamy texture is perfect. Russets can work, but they’re a bit more starchy.
• Fresh Thyme (2 tablespoons, leaves only): Adds a fragrant, earthy note. Dried thyme can fill in (use 2 teaspoons), but fresh is best.
• Gruyère Cheese (1 cup / 100g, shredded): Melts beautifully and adds nuttiness. Fontina or sharp white cheddar are tasty swaps.
• Olive Oil (2 tablespoons): For caramelizing the onions.
• Unsalted Butter (2 tablespoons): Adds richness to the onions. Use vegan butter if needed.
• Heavy Cream (2 tablespoons): Helps achieve a creamy potato layer. Whole milk or oat cream can substitute.
• Egg (1 large, beaten): For brushing the pastry edges to get that gorgeous, golden finish.
• Kosher Salt & Freshly Ground Black Pepper: To season the layers. Taste as you go—potatoes need a good pinch!
• Optional Add-Ins:
  • Chives (2 tablespoons, finely chopped): For garnish and a little oniony zing.
  • Crumbled Goat Cheese (1/4 cup / 30g): If you want a tangy kick.
  • Smoked Paprika (1/2 teaspoon): For a hint of warmth.

Ingredient notes: For best results, slice your potatoes as thin as possible (a mandoline is handy, but a sharp knife and patience work too). If you’re dairy-free, swap in a vegan cheese and plant-based milk or cream. Gluten-free? Try a gluten-free puff pastry if you can find one. In summer, I’ve swapped thyme for fresh rosemary or basil—delicious!

Equipment Needed

• Large Skillet or Sauté Pan: Essential for caramelizing onions. I use a heavy-bottomed skillet for even browning.
• Baking Sheet (about 13×9 inches / 33×23 cm): Lined with parchment for easy cleanup. A rimmed sheet is best to catch any cheesy drips.
• Sharp Knife or Mandoline: For thinly slicing potatoes. I’ve used both—a mandoline makes uniform slices, but a sharp chef’s knife is just fine.
• Parchment Paper: Prevents sticking and makes transferring the pie a breeze.
• Pastry Brush: For brushing egg wash onto the pastry. In a pinch, use the back of a spoon.
• Mixing Bowl: For tossing the potatoes with cream and seasoning.
• Measuring Cups and Spoons: For accuracy (especially with cheese and cream).

If you don’t have a mandoline, just slice the potatoes as thin as you can by hand. Always use a stable cutting board—I once tried to rush and ended up with uneven slices. For cleaning, soak the skillet right after caramelizing—the sugars can get a bit sticky. Budget tip: If you’re just starting out, dollar store sheet pans work fine, but invest in a sturdy one for even baking (I learned the hard way that warped pans can spill your pie!).

How to Make Caramelized Onion Potato Thyme Slab Pie

1. Preheat and Prep: Heat your oven to 400°F (200°C). Line a rimmed baking sheet (about 13×9 inches / 33×23 cm) with parchment paper. Thaw your puff pastry according to package directions. (Tip: Don’t let it get too soft or it’ll be tricky to handle.)
2. Caramelize the Onions (25-30 minutes): Heat 2 tablespoons olive oil and 2 tablespoons butter in a large skillet over medium-low. Add the sliced onions and a pinch of salt. Cook, stirring every few minutes, until the onions are deep golden and jammy—about 25-30 minutes. If they start to stick, add a splash of water. (Patience is key here! Rushing leads to burnt, bitter onions.)
3. Prep the Potatoes: While the onions cook, peel and slice the potatoes as thinly as possible (aim for about 1/8 inch / 3 mm). Toss them in a large bowl with 2 tablespoons heavy cream, salt, and pepper. Make sure every slice is coated.
4. Roll Out the Puff Pastry: On a lightly floured surface, gently roll the two puff pastry sheets together to fit your baking sheet (about 1/4 inch / 6 mm thick). Place on the parchment-lined sheet. If the pastry cracks, just press it back together—no big deal.
5. Layer the Fillings: Start with a layer of caramelized onions, spreading them evenly over the pastry, leaving a 1-inch (2.5 cm) border. Next, arrange the potato slices over the onions, overlapping slightly. Sprinkle with shredded Gruyère and scatter the thyme leaves over the top.
6. Fold and Egg Wash: Fold the pastry edges up and over the filling, pinching the corners to seal. Brush the exposed pastry with beaten egg for that shiny golden crust.
7. Bake (25-30 minutes): Place the slab pie in the oven and bake for 25-30 minutes, or until the pastry is deeply golden and crisp, and the potatoes are fork-tender. Rotate the pan halfway through for even browning.
8. Cool and Slice: Let the pie cool for 5-10 minutes—it sets up and slices more neatly. Scatter with extra thyme or chives if you like. Slice into squares and serve warm or at room temp.

Notes: If your pastry puffs up a lot during baking, gently press it down with a spatula. Don’t stress about perfect layers—rustic looks (and tastes) amazing. If you notice the edges browning too quickly, cover them with foil for the last 10 minutes. I’ve found that slightly overlapping the potato slices helps them cook evenly and keeps the filling creamy.

Cooking Tips & Techniques

• Get Those Onions Right: Caramelizing onions takes time—don’t rush it! Medium-low heat is your friend. If they start looking dry, add a splash of water. I once tried speeding up by cranking the heat, and all I got was burnt onions and disappointment.
• Thin Potato Slices Matter: Uniform, thin slices cook through evenly and meld into the filling. If you’re using a knife, take your time. Mandolines are great, but use the guard to avoid any kitchen mishaps (I’ve learned the hard way).
• Cold Pastry = Flaky Pastry: Keep your puff pastry as cold as possible before baking. If it gets too soft, pop it in the fridge for a few minutes. Warm pastry can get sticky and lose its structure.
• Don’t Skimp on Seasoning: Potatoes need a solid pinch of salt and pepper. Taste the cream and potato mixture before layering to make sure it’s just right.
• Cheese Choices: Gruyère is classic, but don’t be afraid to experiment. I once used sharp cheddar and even a mix of Swiss and mozzarella—delicious!
• Egg Wash for Shine: Brush the pastry edges generously for that bakery-worthy look.
• Multi-tasking: While onions caramelize, prep everything else to save time. If you’re like me and always multi-task, just don’t forget to stir the onions!

Consistency is key—stick to the method and you’ll get a crisp, golden slab pie every time. If something goes wrong (like undercooked potatoes), just pop it back in the oven with a bit of foil over the edges. I’ve made this when distracted by kids, pets, or a podcast—still turns out great.

Variations & Adaptations

• Vegan Version: Use vegan puff pastry (check the label), plant-based butter, cream, and cheese. The flavors hold up beautifully—just make sure your potatoes are well-seasoned.
• Seasonal Veggies: Add thinly sliced zucchini, roasted red peppers, or even a handful of baby spinach. In spring, I love swapping in asparagus tips or leeks.
• Different Cheeses: Not a Gruyère fan? Try sharp white cheddar, smoked gouda, or crumbled goat cheese for a tangy twist. The cheese layer is forgiving—use what you love.
• Herb Swaps: No thyme? Fresh rosemary, oregano, or even sage work well. In summer, a sprinkle of basil at the end is incredible.
• Gluten-Free Option: Use gluten-free puff pastry (Schar makes a good one), and double check your other ingredients.
• Personal Touch: I once added thin slices of fennel and it brought a lovely, mild sweetness. Play with the filling—there’s a lot of wiggle room for your favorites.

For a breakfast vibe, crack a few eggs on top during the last 10 minutes of baking. If allergies are a concern, skip the cheese or use a nut-based alternative. Honestly, this slab pie is a blank canvas—make it yours!

Serving & Storage Suggestions

Serve your Caramelized Onion Potato Thyme Slab Pie warm, fresh from the oven, or at room temperature. I love it with a simple green salad tossed in a lemony vinaigrette or alongside a bowl of tomato soup for a cozy dinner. If you’re feeling fancy, pair it with a crisp white wine or sparkling cider.

To store, let the pie cool completely, then cover tightly and refrigerate for up to 3 days. The flavors actually meld and deepen overnight! For freezing, cut into slices, wrap each piece in foil or parchment, and pop into a freezer bag. They’ll keep for about a month.

Reheat in a 350°F (175°C) oven for 10-15 minutes, or until warmed through and the pastry is re-crisped (microwaving works in a pinch, but you’ll lose some crunch). Pro tip: A quick toast in the air fryer revives the crust perfectly. Don’t be surprised if you find yourself snacking on leftovers straight from the fridge—still delicious!

Nutritional Information & Benefits

This slab pie is hearty and satisfying without feeling heavy. Each serving (about 1/6 of the pie) provides roughly:

• Calories: 340
• Protein: 8g
• Fat: 21g
• Carbs: 29g
• Fiber: 3g

Potatoes offer potassium and vitamin C, while onions bring antioxidants and prebiotics. Using fresh herbs like thyme adds both flavor and a little boost of micronutrients. The recipe is vegetarian and can be adapted for gluten-free and dairy-free diets. Just keep an eye out for allergens in your pastry and cheese. Personally, I love how this pie leaves me feeling full and happy, not weighed down.

Conclusion

If you’re searching for a dish that’s as impressive as it is easy, this Caramelized Onion Potato Thyme Slab Pie is the answer. The combination of sweet onions, creamy potatoes, and flaky pastry is pure comfort, and the recipe is flexible enough to suit any taste or occasion. I’ve made this pie for friends, family, and even potlucks—it always disappears fast!

Don’t be afraid to riff on the ingredients or switch up the herbs and cheese. That’s the beauty of this recipe—it’s a canvas for your creativity. And honestly, that golden, crispy crust is reason enough to make it again and again.

Give it a try, and let me know how you make it your own! Share your tweaks, your stories, or even your kitchen mishaps in the comments. I’d love to hear from you—and I hope this recipe brings a little extra joy (and flavor!) to your table.

Frequently Asked Questions

Can I make this Caramelized Onion Potato Thyme Slab Pie ahead of time?

Yes, you can assemble the pie a few hours ahead, cover, and refrigerate until ready to bake. Bake just before serving for the crispiest pastry.

What’s the best way to slice potatoes thinly for this recipe?

A mandoline slicer works best for uniform, thin slices, but a sharp knife and steady hand does the trick too. Just aim for 1/8 inch (3 mm) if possible.

Can I use sweet potatoes instead of Yukon Golds?

Absolutely! Sweet potatoes add a lovely sweetness and bake up just as nicely. You may want to reduce the caramelized onions slightly, since both are sweet.

How do I keep the puff pastry from getting soggy?

Make sure the onions are cooked down and not watery, and don’t overload the pie with cream. Bake until the pastry is deep golden all over, especially the bottom.

Is this recipe freezer-friendly?

Yes! Slice the cooled pie, wrap pieces tightly, and freeze. Reheat in the oven for best results—makes for a quick, delicious lunch or snack.

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