Step-by-step instructions

1. Preheat your oven to 400°F / 200°C and line a baking sheet with parchment paper.
2. In a large skillet, combine the butter, olive oil, and salt over medium-low heat.
3. Add the sliced onions, cooking them slowly until they become deeply caramelized; this should take about 20-30 minutes.
4. Meanwhile, unfold the puff pastry on the prepared baking sheet. Score a border about an inch from the edge and prick the center with a fork to prevent it from puffing up too much.
5. Once the onions are caramelized, spread them evenly inside the border of the puff pastry.
6. Sprinkle the grated Gruyère cheese and fresh thyme leaves over the onions.
7. Brush the edges of the pastry with the beaten egg to give it a lovely golden finish.
8. Bake for 20–25 minutes until the pastry is golden brown and puffed.
9. Let it cool slightly, then cut it into squares and enjoy while warm.

Best ways to enjoy it

Serve your Caramelized Onion & Gruyère Puff Pastry Slab warm, garnished with a few extra thyme leaves for color. Pair it with a fresh salad dressed in lemon vinaigrette or a side of roasted vegetables. A glass of chilled white wine or a refreshing iced tea also complements the rich flavors beautifully.

Storage and reheating tips

To keep your leftover puff pastry fresh, cover it tightly with plastic wrap or store it in an airtight container in the fridge for up to 2 days. To reheat, simply place it in a preheated oven at 350°F (175°C) for about 10 minutes until warmed through. For longer storage, you can freeze the unbaked slab after assembling it—just make sure to wrap it well. When ready to bake, add an extra few minutes to the baking time from frozen.

Helpful cooking tips

For perfect caramelization, be patient. Cooking the onions slowly over low heat will develop the rich, sweet flavor you’re after. Stir them occasionally, but don’t rush the process. Also, ensure your puff pastry is properly thawed before using; this will help it rise beautifully in the oven.

Creative twists

Feel free to get creative with this recipe! Consider adding other ingredients like sautéed mushrooms for an earthy flavor, or even crispy bacon or pancetta for a meaty addition. If you’re looking for a vegetarian alternative, swap the Gruyère for a dairy-free cheese. You could also experiment with different herbs, such as adding sage or chives to mix things up.

FAQs

What is the prep time for this recipe?

The total prep time is about 40 minutes, with 20-30 minutes of cooking time for the onions.

Can I use a different type of cheese?

Absolutely! While Gruyère adds a unique flavor, you can substitute it with cheeses like Swiss, Fontina, or even a sharp cheddar for a different twist.

How do I know when the onions are caramelized?

They should be a deep golden brown color and have a sweet aroma. The process takes time, typically 20-30 minutes over medium-low heat, but it’s worth the wait!

This Caramelized Onion & Gruyère Puff Pastry Slab with Thyme is sure to become a favorite in your recipe collection, perfect for any occasion and consistently mouthwatering.