French Onion Puff Pastry Tart (Printable)

Crisp puff pastry crowned with caramelized onions, melted Gruyère and thyme for a savory French-style tart.

# What You Need:

→ Tart

01 - 1 sheet puff pastry (about 9 oz), thawed if frozen
02 - 3 large yellow onions, thinly sliced
03 - 2 tablespoons unsalted butter
04 - 1 tablespoon 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
09 - 1 cup grated Gruyère (approximately 4.2 oz)
10 - 1 large egg, beaten (for egg wash)

→ Garnish

11 - Fresh thyme sprigs (optional)
12 - Freshly ground black pepper (optional)

# Steps:

01 - Preheat oven to 400°F. Line a baking sheet with parchment paper and set aside.
02 - In a large skillet over medium heat, melt the butter with the olive oil. Add the sliced onions, sugar, salt and pepper and cook, stirring frequently, until deeply golden and caramelized, about 20–25 minutes. Stir in the 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 a roughly 10 x 14 inch rectangle. Transfer the dough to the prepared baking sheet.
04 - Using a sharp knife, score a 1/2-inch border around the edge of the pastry without cutting through. Prick the interior area all over with a fork to prevent excessive rising.
05 - Brush the scored border with the beaten egg to promote a glossy, golden finish.
06 - Evenly spread the caramelized onions within the scored border, then sprinkle the grated Gruyère over the onions.
07 - Bake in the preheated oven for 18–22 minutes, or until the pastry is fully puffed and deep golden brown at the edges.
08 - Remove from the oven and let rest for about 5 minutes. Garnish with extra thyme and a crack of black pepper if desired. Slice and serve warm or at room temperature.

# Expert Advice:

01 -
  • The layers of sweet caramelized onions and nutty Gruyère melt perfectly into the crackly pastry, stealing the spotlight at any table.
  • You can prep all the bits ahead, then just assemble and bake when you need that effortless wow factor.
02 -
  • The first time I tried rushing the onions, they burned at the edges but stayed pale in the middle—low and slow is non-negotiable.
  • Pricking the pastry inside the border keeps the center pleasantly crisp, preventing a soggy bottom disaster.
03 -
  • Use a heavy-bottomed skillet for the onions—a thin pan scorches instead of caramelizing.
  • An extra sprinkle of thyme right after baking brings out the most amazing aroma—it’s worth the tiny bit of extra effort.
Go Back