Caramelized Onion Rustic Tart (Printable)

Savory tart topped with golden caramelized onions, Gruyère, and fresh thyme on crisp puff pastry.

# What You Need:

→ Pastry

01 - 1 sheet frozen puff pastry, thawed (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 - Optional: 1 tbsp chopped fresh chives for garnish

# How-To Steps:

01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - Melt butter and olive oil in a large skillet over medium heat. Add sliced onions, sugar, salt, and pepper. Cook, stirring frequently, until onions turn deep golden and caramelize, about 25 to 30 minutes.
03 - On a lightly floured surface, roll puff pastry into 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 uncovered.
05 - Sprinkle half the grated Gruyère over the crème mixture. Distribute caramelized onions over cheese and sprinkle with thyme leaves. Top with remaining Gruyère.
06 - Fold the edges of the pastry gently over the filling to form a rustic border.
07 - Bake the tart for 20 to 25 minutes, or until pastry is golden brown and crisp. Allow to cool slightly before slicing.
08 - Optionally, garnish with chopped fresh chives. Serve warm or at room temperature.

# Expert Suggestions:

01 -
  • It uses just one sheet of pastry and a handful of everyday ingredients, but tastes like you ordered it from a French bistro.
  • The onions do all the work, turning sweet and jammy while you barely have to babysit them.
  • It's one of those rare dishes that works hot from the oven, at room temperature, or even cold from the fridge the next morning.
  • You can dress it up with wine and cloth napkins or eat it straight off the pan with your fingers—no judgment either way.
02 -
  • Don't rush the onions—if they start to brown too fast, lower the heat and add a splash of water to slow things down.
  • If your pastry is soggy in the middle, your oven might be running cool—use an oven thermometer or bump the temp up by 10 degrees next time.
  • Leaving that one-inch border is crucial—it puffs up and holds everything in, so don't skip it.
03 -
  • Use a cast iron skillet for caramelizing if you have one—it holds heat evenly and gives you better color on the onions.
  • If you're making this ahead, caramelize the onions the night before and store them in the fridge—assembly takes five minutes the next day.
  • Don't skip the parchment paper—it makes sliding the tart off the pan effortless and saves you from scrubbing stuck-on cheese.
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