Caramelized Onion Rustic Tart (Printable)

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

# What You’ll Need:

→ Pastry

01 - 1 sheet frozen puff pastry, thawed (approx. 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 (4 fl oz)
09 - 1 teaspoon Dijon mustard
10 - 1/2 cup grated Gruyère cheese (2.1 oz)
11 - 1 teaspoon fresh thyme leaves or 1/2 teaspoon dried thyme
12 - Optional: 1 tablespoon chopped fresh chives for garnish

# How-To Steps:

01 - Preheat the oven to 400°F and 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 the puff pastry to a 12-inch square or circle. Transfer it onto the prepared baking sheet.
04 - Combine crème fraîche and Dijon mustard in a small bowl. Spread evenly over the pastry, leaving a 1-inch border.
05 - Sprinkle half the Gruyère over the crème mixture. Evenly distribute the caramelized onions on top, then sprinkle with thyme. Finish with remaining Gruyère cheese.
06 - Fold the edges of the pastry gently over the filling to form 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 chopped chives. Serve warm or at room temperature.

# Expert Hints:

01 -
  • The caramelized onions become sweet and savory, creating a depth of flavor that feels indulgent despite the humble ingredients
  • It looks impressive coming out of the oven but comes together with minimal effort and basic pantry staples
02 -
  • Rushing the onions is the biggest mistake. Low and slow gives you that deep, jammy sweetness that makes the tart special.
  • Adding a splash of balsamic vinegar during the last five minutes of caramelizing creates a subtle complexity people will notice but not quite identify.
03 -
  • Use a mandoline or very sharp knife to slice the onions thinly and evenly. Thin slices cook faster and caramelize more uniformly.
  • Brush the folded pastry edges with a little beaten egg before baking for an extra golden, professional looking finish.
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