Caramelized Onion Rustic Tart (Printable)

Savory tart with flaky pastry topped by deep golden caramelized onions and melted Gruyère.

# What You'll Need:

→ Pastry

01 - 1 sheet frozen puff pastry, thawed (approximately 8.8 oz)

→ Onions

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 salt
07 - 1/4 teaspoon freshly ground black pepper

→ Filling & Topping

08 - 1/2 cup crème fraîche or sour cream
09 - 1 teaspoon Dijon mustard
10 - 1/2 cup grated Gruyère cheese (about 2.1 oz)
11 - 1 teaspoon fresh thyme leaves or 1/2 teaspoon dried thyme
12 - Optional: 1 tablespoon chopped fresh chives for garnish

# Step-by-step Directions:

01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper.
02 - Melt butter with olive oil in a large skillet over medium heat. Add sliced onions, sugar, salt, and pepper. Cook, stirring frequently, until onions are deeply golden and caramelized, about 25 to 30 minutes.
03 - On a lightly floured surface, roll out the puff pastry to a 12-inch square or circle. Transfer it to the prepared baking sheet.
04 - Combine crème fraîche with Dijon mustard in a small bowl and spread evenly over the pastry, leaving a 1-inch border uncovered.
05 - Sprinkle half of the grated Gruyère over the crème mixture. Evenly distribute the caramelized onions atop the cheese and scatter thyme leaves over. Finish by sprinkling the remaining Gruyère cheese on top.
06 - Gently fold the edges of the pastry over the filling to create a rustic border.
07 - Bake in the preheated oven for 20 to 25 minutes until the pastry is golden brown and crisp. Let cool slightly before serving.
08 - Optionally garnish with chopped fresh chives. Serve warm or at room temperature.

# Expert Advice:

01 -
  • The pastry turns impossibly crispy while the onions stay silky and melt on your tongue.
  • It works equally well as a fancy appetizer or a simple weeknight dinner with a salad.
  • Once the onions are done, assembly feels more like play than cooking.
02 -
  • Don't rush the onions—low and slow caramelization is what transforms them from sharp to sweet, and no shortcut gets you there.
  • If your pastry edge browns too quickly before the center cooks, tent a loose piece of foil over just the edges and continue baking.
  • Crème fraîche is milder and richer than sour cream, but both work; if using sour cream, use a tiny bit less since it's tangier.
03 -
  • If you're short on time, buy pre-sliced onions from the produce section—they'll save 10 minutes and still caramelize beautifully.
  • Leftover tart reheats gently in a 300°F oven and tastes just as good the next day, making it perfect for meal prep.
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