Caramelized Onion Rustic Tart (Printable Version)

Flaky pastry topped with golden onions, creamy cheese, and herbs for a rustic, savory dish.

# What You 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
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

# How to Make It:

01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - In a large skillet over medium heat, melt butter with olive oil. Add sliced onions, sugar, salt, and pepper. Cook, stirring frequently, until onions turn deeply golden and caramelized, about 25 to 30 minutes.
03 - Roll puff pastry on lightly floured surface into a 12-inch square or circle. Transfer to prepared baking sheet.
04 - In a small bowl, combine crème fraîche and Dijon mustard. Spread mixture evenly over pastry, leaving a 1-inch border clear.
05 - Scatter half of the Gruyère evenly over the cream layer. Distribute caramelized onions on top, then sprinkle with thyme. Finish with remaining Gruyère cheese.
06 - Fold pastry edges gently inward over filling to form a rustic border.
07 - Bake for 20 to 25 minutes until pastry is golden and crisp. Allow to cool slightly before slicing.
08 - If desired, garnish with fresh chopped chives. Serve warm or at room temperature.

# Expert Advice:

01 -
  • Those caramelized onions taste like you've been stirring them for hours, but really it's just patience and butter doing the work.
  • It looks fancy enough to serve at dinner, but it's honestly easier than most weeknight meals.
  • The crispy-flaky pastry edges are so good you'll find yourself picking at the tart long after it's cooled.
02 -
  • Caramelizing onions really does take 25+ minutes, and there's no shortcut that doesn't sacrifice flavor—rushing them means you'll lose that deep sweetness that makes the whole thing work.
  • Don't skip the mustard; I learned this by accident the first time I made this without it, and the tart was good but felt like it was missing something crucial underneath.
  • The pastry edges will brown faster than the center, so if they're getting dark before the filling is set, drape a piece of foil loosely over the top.
03 -
  • Let your puff pastry thaw completely at room temperature—about 30 minutes—so it rolls without cracking, but not so long that it gets warm and sticky.
  • If your oven runs hot, rotate the tart halfway through baking so the pastry browns evenly without the edges getting too dark.
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