French Onion Puff Pastry Tart (Printer View)

Thin, flaky puff pastry layered with caramelized onions, melted Gruyère and thyme for a savory, golden tart.

# What you'll need:

→ For the Tart

01 - 1 sheet (about 250 g) puff pastry, thawed if frozen
02 - 3 large yellow onions, thinly sliced
03 - 2 tbsp unsalted butter
04 - 1 tbsp olive oil
05 - 1 tsp sugar
06 - 1/2 tsp salt
07 - 1/4 tsp freshly ground black pepper
08 - 1 tsp fresh thyme leaves (plus extra for garnish)
09 - 120 g Gruyère cheese, grated
10 - 1 egg, beaten (for egg wash)

→ Garnish (optional)

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

# Method:

01 - Preheat the oven to 400°F (200°C). 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 often, for 20–25 minutes until the onions are deeply golden and caramelized. Stir in the thyme leaves during the last 5 minutes of cooking. Remove from heat and let cool slightly.
03 - Roll out the puff pastry on a lightly floured surface to a rectangle about 10 x 14 inches (25 x 35 cm). Transfer to the prepared baking sheet.
04 - Using a sharp knife, score a 1/2-inch (1.5 cm) border around the edge of the pastry, being careful not to cut all the way through. Prick the inside area all over with a fork.
05 - Brush the border with beaten egg.
06 - Evenly spread the caramelized onions inside the border. Sprinkle the Gruyère cheese over the onions.
07 - Bake for 18–22 minutes or until the pastry is puffed and golden brown.
08 - Remove from the oven and let cool for 5 minutes. Garnish with extra thyme and black pepper if desired. Slice and serve warm or at room temperature.

# Expert Advice:

01 -
  • No one expects something with so few ingredients to taste so rich—you’ll look like a kitchen magician.
  • This tart is barely fuss and all flavor, perfect when you crave classic comfort but don’t want to labor over soup.
02 -
  • I once rushed the onions and regretted it—patience truly creates the deepest, richest flavor.
  • Chilling the rolled pastry on the baking sheet for five minutes before assembling gives you those crispest layers.
03 -
  • Clean your skillet as soon as the onions are out to avoid a caramelized mess later—learned that the sticky way.
  • Try using a pastry wheel for a decorative edge on the tart—my not-so-secret for impressing guests.
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