Caramelized Onion Rustic Tart (Printable)

Savory tart with golden caramelized onions and flaky pastry for light meals or appetizers.

# What You Need:

→ Pastry

01 - 1 sheet frozen puff pastry, thawed (about 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 (120 ml)
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. 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 are deeply golden and caramelized, approximately 25 to 30 minutes.
03 - Roll the thawed puff pastry on a lightly floured surface into a 12-inch square or circle. Transfer the pastry to the prepared baking sheet.
04 - In a small bowl, combine crème fraîche and Dijon mustard. Spread this mixture evenly over the pastry, leaving a 1-inch border around the edges.
05 - Scatter half of the grated Gruyère over the crème mixture, then evenly top with caramelized onions and sprinkle with thyme leaves. Finish by adding the remaining Gruyère cheese.
06 - Carefully fold the edges of the pastry over the filling to form a rustic border.
07 - Bake for 20 to 25 minutes until the pastry is golden brown and crisp. Allow to cool slightly before cutting.
08 - If desired, garnish with chopped fresh chives. Serve warm or at room temperature.

# Expert Suggestions:

01 -
  • The caramelization does most of the work while you read or stand there philosophizing about dinner.
  • It feels fancy enough for guests but honest enough for yourself on a quiet night.
  • Crispy pastry meets silky onions in a way that somehow tastes more sophisticated than the effort requires.
02 -
  • Don't rush the onion caramelization—low heat and patience make them sweet and jammy; high heat just makes them brown on the outside and bitter.
  • If your pastry edges brown too quickly, tent them with foil for the last 10 minutes of baking so they don't burn.
03 -
  • If your pastry seems to be browning too fast, slide the baking sheet to a lower rack or reduce the oven temperature by 25°F—every oven is different, and you want the filling bubbling while the pastry is still golden.
  • Make sure your onions are truly caramelized and not just softened—taste one if you're unsure, and if it still tastes sharp or raw, give them another 5 minutes.
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