Caramelized Onion Rustic Tart (Printable)

Flaky pastry crowned with caramelized onions, Gruyère, and fresh thyme in a savory rustic tart.

# What You'll Need:

→ Pastry

01 - 1 sheet frozen puff pastry, thawed (approx. 9 oz)

→ Onions

02 - 3 large yellow onions, thinly sliced
03 - 2 tbsp unsalted butter
04 - 1 tbsp olive oil
05 - 1 tsp granulated sugar
06 - 1/2 tsp salt
07 - 1/4 tsp freshly ground black pepper

→ Filling & Topping

08 - 1/2 cup crème fraîche or sour cream
09 - 1 tsp Dijon mustard
10 - 1/2 cup grated Gruyère cheese (2 oz)
11 - 1 tsp fresh thyme leaves or 1/2 tsp dried thyme
12 - Optional: 1 tbsp chopped fresh chives for garnish

# Directions:

01 - Preheat oven to 400°F and 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, about 25 to 30 minutes.
03 - Roll out the thawed puff pastry on a lightly floured surface to a 12-inch square or circle. Transfer it to the prepared baking sheet.
04 - In a small bowl, mix crème fraîche and Dijon mustard. Spread evenly over the pastry, leaving a 1-inch border around the edges.
05 - Sprinkle half of the grated Gruyère over the cream mixture. Distribute the caramelized onions evenly on top and sprinkle with thyme leaves. Finish by scattering the remaining Gruyère cheese.
06 - Fold the edges of the pastry gently 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 briefly before cutting.
08 - Optionally garnish with fresh chopped chives. Serve warm or at room temperature.

# Expert Tips:

01 -
  • It looks fancy enough to serve guests but honest enough to eat alone with a glass of wine on a Tuesday.
  • Frozen puff pastry does the heavy lifting, so you can focus on the magic of caramelization without stress.
  • The contrast between crispy pastry and silky, sweet onions feels like a small luxury that costs almost nothing.
02 -
  • Caramelizing onions cannot be rushed—medium heat and patience matter more than high heat and speed, otherwise they'll burn and taste bitter.
  • If your oven runs hot, start checking the pastry at 18 minutes so the edges don't brown too quickly.
  • A splash of balsamic vinegar in the last 5 minutes of onion cooking adds a subtle depth that makes people ask what your secret is.
03 -
  • If your onions are browning too fast, lower the heat—slow and gentle beats rushed and scorched every time.
  • Don't thaw puff pastry at room temperature for hours; thaw it in the fridge overnight or on the counter for 20 minutes so it stays flaky and cooperative.
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