French Onion Puff Pastry Tart (Print Version)

Flaky puff pastry topped with caramelized onions, Gruyère and thyme for a savory, shareable tart.

# What You'll Need:

→ Tart

01 - 1 sheet puff pastry (about 8.8 oz), thawed if frozen
02 - 3 large yellow onions, thinly sliced
03 - 2 tablespoons unsalted butter
04 - 1 tablespoon extra-virgin olive oil
05 - 1 teaspoon granulated sugar
06 - 1/2 teaspoon fine salt
07 - 1/4 teaspoon freshly ground black pepper
08 - 1 teaspoon fresh thyme leaves, plus extra for garnish
09 - 4.2 ounces Gruyère, grated (about 1 cup)
10 - 1 large egg, beaten (for egg wash)

→ Garnish (optional)

11 - Extra fresh thyme sprigs
12 - Freshly ground black pepper

# How-To Steps:

01 - Preheat oven to 400°F. Line a baking sheet with parchment paper and set aside.
02 - Heat a large skillet over medium; add butter and olive oil. Add sliced onions, sugar, salt and pepper. Cook, stirring frequently, for 20–25 minutes until deeply golden and jammy. Stir in thyme during the final 5 minutes. Remove from heat and allow to cool slightly.
03 - On a lightly floured surface, roll the puff pastry to roughly a 10 x 14-inch rectangle. Transfer the sheet to the prepared baking sheet.
04 - Using a sharp knife, score a 1/2-inch border around the perimeter without cutting through. Prick the inner field all over with a fork to prevent excessive rising.
05 - Brush the scored border with the beaten egg to promote color and shine.
06 - Spread the caramelized onions evenly inside the border, then scatter the grated Gruyère over the onions.
07 - Bake for 18–22 minutes, or until the pastry is fully puffed and deep golden brown.
08 - Remove from oven and rest 5 minutes. Garnish with extra thyme and a crack of black pepper. Slice and serve warm or at room temperature.

# Expert Advice:

01 -
  • The deeply golden onions practically melt into the buttery pastry, making each bite addictive.
  • It always impresses guests yet takes surprisingly little effort to prepare.
02 -
  • If you rush the onions over high heat, they'll burn instead of caramelize—slow and steady wins here.
  • Layering cheese over still-warm onions helps it melt beautifully without overbaking the crust.
03 -
  • If your pastry begins to brown too quickly, tent the edges with foil to protect them.
  • A splash of white wine added near the end of caramelizing gives the onions irresistible depth—just don't rush it.
Go Back