French Onion Puff Pastry Tart (Print View)

Thin, crispy puff pastry topped with caramelized onions, nutty Gruyère and fresh thyme.

# Components:

→ 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, plus extra for garnish
09 - 1 cup grated Gruyère (about 4.25 oz)
10 - 1 large egg, beaten (for egg wash)

→ Garnish (optional)

11 - Fresh thyme sprigs
12 - Freshly ground black pepper

# Directions:

01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper.
02 - In a large skillet over medium heat, melt the butter with the olive oil. Add the sliced onions, sugar, salt and pepper. Cook, stirring frequently, for 20–25 minutes until the onions are deep golden and caramelized. Stir in the thyme leaves in the final 5 minutes, then remove from heat and let cool slightly.
03 - On a lightly floured surface, roll the puff pastry to a rectangle about 10 by 14 inches. Transfer the sheet to the prepared baking sheet.
04 - Using a sharp knife, score a 1/2-inch border around the edge of the pastry without cutting all the way through. Prick the interior area evenly with a fork to dock.
05 - Brush the scored border with the beaten egg to promote browning and aid rise.
06 - Spread the caramelized onions evenly inside the scored border, then sprinkle the grated Gruyère over the onions in an even layer.
07 - Bake in the preheated oven for 18–22 minutes, or until the pastry is fully puffed and a deep golden brown.
08 - Remove from the oven and allow to rest for 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 -
  • Baking this tart fills your kitchen with those savory, irresistible aromas that make people linger just a little longer.
  • It’s so quick and forgiving that you’ll never fear throwing it together, even on a busy weeknight.
02 -
  • If the onions rush or scorch, the whole tart suffers—low, steady heat is key, even if it feels slow.
  • Letting the pastry thaw fully (but not get too warm) prevents frustrating cracks and sticking when rolling out.
03 -
  • Caramelize the onions in a heavy, wide pan for maximum flavor—crowding slows everything down.
  • Slicing the onions as uniformly as possible ensures they cook evenly, so take an extra minute here.
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