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Close-up of dark jammy onions and melted Gruyere on a golden flaky puff pastry crust. (Caramelized Onion Tart)

Caramelized Onion Tart with Bubbly Gruyere and Flaky Crust

This Caramelized Onion Tart features dark, jammy onions and melted, bubbly Gruyere cheese atop a shatteringly flaky puff pastry crust. A rustic and sophisticated savory tart perfect for brunch or elegant entertaining.
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Prep Time 15 minutes
Cook Time 45 minutes
Total Time 1 hour
Servings: 6 slices
Course: Appetizer, Brunch, Lunch
Cuisine: French
Calories: 320
Ingredients Equipment Method Notes

Ingredients
  

Main Ingredients
  • 1 sheet Puff Pastry Thawed but cold (approx. 250g)
  • 1 large Egg For egg wash
  • 3 large Yellow Onions Thinly sliced (approx. 900g)
  • 2 tbsp Salted Butter 30g
  • 1 tbsp Olive Oil 15ml
  • 1.5 cups Gruyere Cheese 150g, freshly shredded
  • 1 tbsp Fresh Thyme Leaves Plus extra sprigs for garnish
  • 0.5 tsp Cracked Black Pepper To taste

Equipment

  • 1 Heavy-bottomed skillet Stainless steel or cast iron preferred.
  • 1 Baking sheet Rimmed sheet for even baking.
  • 1 Parchment paper Prevents sticking and aids presentation.

Instructions
 

Prepare the Onions
  1. In a large skillet, melt butter and olive oil over medium-low heat. Add sliced onions and cook, stirring occasionally, for 35-45 minutes until they are dark brown and jammy.
  2. Remove from heat and let the onions cool slightly before assembling the tart.
Assemble and Bake
  1. Preheat oven to 400°F (200°C). Roll puff pastry onto parchment paper. Score a 1-inch border and dock the center with a fork.
  2. Spread onions inside the border. Top with shredded Gruyere, thyme leaves, and cracked black pepper.
  3. Brush edges with egg wash. Bake for 15-20 minutes until the edges are golden-brown and the cheese is bubbly and slightly browned.

Notes

Always use fresh thyme for the best aromatic flavor profile.
If onions dry out too fast while cooking, add 1 tbsp of water to the pan.
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