Caramelized Onion Rustic Tart (Printable Version)

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

# What You'll Need:

→ Pastry

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

→ 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 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 - 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 - On a lightly floured surface, roll out the thawed puff pastry to a 12-inch square or circle. Transfer it to the prepared baking sheet.
04 - In a small bowl, combine crème fraîche and Dijon mustard. Spread the mixture evenly over the puff pastry, leaving a 1-inch border around the edges.
05 - Sprinkle half the grated Gruyère over the crème mixture. Layer the caramelized onions on top, then sprinkle with thyme. Finish with the remaining Gruyère cheese.
06 - Gently fold the edges of the pastry over the filling to create a rustic border.
07 - Bake in the preheated oven for 20 to 25 minutes, or until the pastry is golden and crisp. Remove from oven and let cool slightly before slicing.
08 - If desired, garnish with chopped fresh chives. Serve warm or at room temperature.

# Expert Tips:

01 -
  • Caramelization sounds fancy but it's truly just patient stirring—no technique required, just time and butter.
  • The whole tart comes together in about an hour, making it perfect for a weeknight dinner that feels special.
  • It works just as well cold from the fridge as it does warm, which means you can make it ahead and still impress.
02 -
  • Don't rush the caramelization—those 25 to 30 minutes of regular stirring are non-negotiable if you want deep flavor instead of half-cooked onions.
  • If your onions ever start to stick or brown too fast, lower the heat slightly and add a splash of water to deglaze the pan.
  • Make sure your oven is fully preheated before baking, otherwise the pastry won't puff properly.
03 -
  • Keep your heat at medium while caramelizing—high heat cooks the onions too fast on the outside and leaves them raw inside.
  • Taste the caramelized onions before putting them on the tart; if they taste sharp or acidic instead of sweet, they need more time.
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