Caramelized Onion Rustic Tart (Printable)

A rustic tart topped with deeply caramelized onions and melting Gruyère on flaky puff pastry.

# Ingredient List:

→ Pastry

01 - 1 sheet frozen puff pastry, thawed (approximately 8.8 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 (4 fl oz)
09 - 1 tsp Dijon mustard
10 - 1/2 cup grated Gruyère cheese (2.1 oz)
11 - 1 tsp fresh thyme leaves or 1/2 tsp dried thyme
12 - 1 tbsp chopped fresh chives (optional, for garnish)

# Directions:

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 - On a lightly floured surface, roll out the puff pastry to a 12-inch square or circle. Transfer to the prepared baking sheet.
04 - Combine crème fraîche and Dijon mustard in a small bowl. Spread mixture evenly over pastry, leaving a 1-inch border around the edges.
05 - Sprinkle half the Gruyère cheese over the crème mixture. Evenly distribute caramelized onions on top, then sprinkle with thyme. Finish by adding the remaining Gruyère cheese.
06 - Gently fold pastry edges over filling to form a rustic border.
07 - Bake for 20 to 25 minutes, or until pastry is golden brown and crisp. Allow to cool slightly before slicing.
08 - Optional: garnish with chopped fresh chives. Serve warm or at room temperature.

# Expert Tips:

01 -
  • The whole thing comes together in just an hour, but tastes like you've been laboring in a French kitchen all day.
  • Those caramelized onions are sweet, savory, and deeply satisfying—they're the real star here, and they'll change how you think about humble onions forever.
  • It's vegetarian, impressive enough for guests, and somehow elegant enough for a quiet dinner for one.
02 -
  • Don't skip the sugar or turn the heat too high trying to rush the caramelization—low, patient heat is what transforms ordinary onions into liquid gold, and it genuinely takes about 25 to 30 minutes.
  • Keep the pastry cold right up until it goes into the oven; if it gets warm and soft, pop the whole baking sheet in the freezer for 10 minutes and it'll bounce back to flaky perfection.
  • The crème fraîche base is essential—it keeps the pastry from getting soggy from the onion's moisture while adding a cool, creamy contrast to all that caramelized sweetness.
03 -
  • If your caramelized onions seem to still have their sharp edge after 25 minutes, they're not ready yet—give them another 5 minutes and taste again; patience here makes all the difference.
  • Serve this with a crisp white wine like Sauvignon Blanc, and watch how the acidity cuts through the richness and makes everything taste even better.
  • Leftovers keep in the fridge for 3 days and are somehow just as good cold or gently reheated in a 300°F oven, making this perfect for meal prep or casual entertaining.
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