Caramelized Onion Rustic Tart (Printable)

Flaky pastry layered with golden onions, creamy crème fraîche, Gruyère cheese, and fragrant thyme.

# What You'll Need:

→ Pastry

01 - 1 sheet frozen puff pastry, thawed (approx. 9 oz)

→ Onions

02 - 3 large yellow onions, thinly sliced
03 - 2 tbsp unsalted butter
04 - 1 tbsp olive oil
05 - 1 tsp granulated sugar
06 - 1/2 tsp salt
07 - 1/4 tsp freshly ground black pepper

→ Filling & Topping

08 - 1/2 cup crème fraîche or sour cream (4 fl oz)
09 - 1 tsp Dijon mustard
10 - 1/2 cup grated Gruyère cheese (2 oz)
11 - 1 tsp fresh thyme leaves or 1/2 tsp dried thyme
12 - Optional: 1 tbsp chopped fresh chives for garnish

# Method:

01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - In a large skillet over medium heat, melt butter and olive oil together. Add sliced onions, sugar, salt, and pepper. Cook, stirring frequently, until onions are deeply golden and caramelized, approximately 25 to 30 minutes.
03 - Roll out the thawed puff pastry on a lightly floured surface into a 12-inch square or round shape. Transfer to the prepared baking sheet.
04 - Combine crème fraîche and Dijon mustard in a small bowl. Spread mixture evenly over pastry, leaving a 1-inch border around the edges.
05 - Sprinkle half of the grated Gruyère cheese over the crème mixture. Evenly distribute the caramelized onions atop, then sprinkle thyme leaves and finish with the remaining Gruyère cheese.
06 - Gently fold the edges of the pastry inward over the filling to create a rustic crust.
07 - Bake in preheated oven for 20 to 25 minutes, or until the pastry is golden brown and crisp. Remove and allow to cool slightly before slicing.
08 - Optionally, sprinkle with chopped fresh chives. Serve warm or at room temperature.

# Expert Advice:

01 -
  • It looks like you spent hours, but the whole thing comes together in about an hour with minimal effort.
  • The sweet, jammy onions paired with tangy crème fraîche and nutty Gruyère create a flavor balance that feels both comforting and refined.
  • You can serve it warm as a main dish or slice it into small squares for an appetizer that disappears fast.
02 -
  • Don't crank up the heat to speed up the onions—they'll burn on the edges and stay raw in the middle instead of turning sweet and soft.
  • If your puff pastry is too cold, it'll crack when you roll it, and if it's too warm, it'll stick and tear, so aim for cool and pliable.
  • Leave that border untopped—it needs to puff up freely, and any filling on the edge will weigh it down and make it soggy.
03 -
  • Use a sharp knife to slice the onions thin and evenly—it helps them cook at the same rate and caramelize beautifully.
  • If you have extra caramelized onions, make a double batch and freeze them in portions—they're gold for sandwiches, pizzas, and pasta.
  • Let the tart rest for five minutes after baking so the filling sets slightly and doesn't slide off when you slice it.
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