Caramelized Onion Tart Quick (Printer-friendly)

Flaky pastry layered with rich caramelized onions and Gruyère cheese, finished with thyme and a touch of Dijon mustard.

# What You'll Need:

→ Pastry

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

→ Onions

02 - 3 large yellow onions, thinly sliced
03 - 2 tablespoons unsalted butter
04 - 1 tablespoon olive oil
05 - 1 teaspoon 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 (2 oz)
11 - 1 teaspoon fresh thyme leaves or 1/2 teaspoon dried thyme
12 - Optional: 1 tablespoon chopped fresh chives for garnish

# How To Make It:

01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper.
02 - Heat butter and 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 - Roll out the puff pastry on a lightly floured surface to a 12-inch square or circle. Transfer the pastry to the prepared baking sheet.
04 - In a small bowl, mix crème fraîche with Dijon mustard. Spread the mixture evenly over the pastry, leaving a 1-inch border around the edges.
05 - Sprinkle half of the grated Gruyère over the crème mixture. Top with the caramelized onions, then scatter thyme leaves evenly. Finish by sprinkling the remaining Gruyère over the top.
06 - Gently fold the edges of the pastry over the filling to create a rustic border.
07 - Bake for 20 to 25 minutes until the pastry is golden and crisp. Allow to cool slightly before slicing.
08 - Optionally garnish with fresh chives. Serve warm or at room temperature.

# Expert Advice:

01 -
  • Golden caramelized onions become naturally sweet and mellow, transforming from humble pantry staple into something luxurious.
  • The whole thing comes together in about an hour, yet tastes like you spent all afternoon in the kitchen.
  • It works equally well as a casual lunch, an elegant appetizer, or even a light dinner with a simple salad alongside.
02 -
  • Don't rush the onions—true caramelization takes time, and speeding it up by turning up the heat just burns them brown without creating that sweet, deep flavor.
  • Keep that pastry border because it protects the edges from over-browning and gives you something gorgeous to sink your teeth into.
  • The tart is actually better made a few hours ahead and served at room temperature, which lets all the flavors settle and makes it easier to slice cleanly.
03 -
  • Keep the oven hot and the pastry cold—the contrast is what creates those shattered, crispy layers that make puff pastry so irresistible.
  • Don't skip the creamy layer under the onions; it acts as a barrier that keeps the pastry from getting soggy while adding a luxurious element.
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