French Onion Puff Pastry Tart (Printable Version)

Thin, crisp puff pastry topped with caramelized onions, Gruyère and thyme — a savory French-inspired tart.

# What You Need:

→ For the 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 (about 4.2 oz)
10 - 1 large egg, beaten (for egg wash)

→ Garnish (optional)

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

# Step-by-Step Directions:

01 - Set the oven to 400°F and line a baking sheet with parchment paper.
02 - Warm a large skillet over medium heat, add butter and olive oil, then add sliced onions, sugar, salt and pepper. Cook, stirring frequently, for 20–25 minutes until deeply golden and soft; stir in thyme during the final five minutes. Remove from heat and let cool slightly.
03 - On a lightly floured surface, roll the puff pastry to roughly a 10×14 inch rectangle and transfer it to the prepared sheet.
04 - Using a sharp knife, score a 1/2-inch border around the edge without cutting through. Prick the interior within the border all over with a fork to prevent excess rising.
05 - Brush the scored border with the beaten egg to promote a golden finish.
06 - Spread the caramelized onions evenly inside the border and sprinkle the grated Gruyère over the onions.
07 - Bake 18–22 minutes or until the pastry is puffed and deeply golden.
08 - Allow to cool 5 minutes on the sheet, then garnish with thyme and a crack of black pepper. Slice and serve warm or at room temperature.

# Expert Advice:

01 -
  • Caramelized onions and buttery pastry are a match that always feels like a little kitchen magic.
  • The tart is wonderfully simple to throw together but tastes as though you spent all day crafting it.
02 -
  • If you try to rush the onions, you’ll never get that deep, jammy color or flavor—it’s worth the patience.
  • Scoring the pastry border sets up a crisp, defined edge that looks beautiful and helps hold the filling in place.
03 -
  • Keep puff pastry cold until right before baking or it won’t rise as prettily.
  • Let onions caramelize patiently for real depth of flavor—don’t turn up the heat to rush them.
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