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Close-up of a Caramelized Onion Tart showing the jammy onion layers and bubbly white cheese topping.

Caramelized Onion Tart with Flaky Puff Pastry and Gruyère

This Caramelized Onion Tart features deeply browned, jammy onions and bubbly toasted Gruyère cheese on a golden-brown, visibly flaky puff pastry crust. Perfect as a sophisticated appetizer or a decadent savory lunch option.
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Prep Time 45 minutes
Cook Time 25 minutes
Total Time 1 hour 10 minutes
Servings: 6 slices
Course: Appetizer, Lunch
Cuisine: French-inspired
Calories: 310
Ingredients Equipment Method Notes

Ingredients
  

Main Ingredients
  • 1 sheet Puff Pastry All-butter, thawed but chilled
  • 1 large Egg Beaten with 1 tsp water for wash
  • 3 large Yellow Onions Thinly sliced (approx. 2 lbs)
  • 2 tbsp Unsalted Butter 30g
  • 1 tbsp Olive Oil 15ml
  • 1.5 cups Gruyère Cheese 150g, shredded
  • 1 tbsp Balsamic Vinegar 15ml
  • 4 sprigs Fresh Thyme Leaves stripped
  • 1 tsp Coarse Sea Salt Flakes To taste

Equipment

  • 1 Heavy-bottomed skillet Cast iron or stainless steel preferred.
  • 1 Baking sheet Standard half-sheet size.
  • 1 Parchment paper To prevent sticking and aid cleanup.

Instructions
 

Prepare the Onions
  1. Melt butter and oil in a skillet over medium-low heat. Add onions and cook, stirring occasionally, until soft and translucent.
  2. Continue cooking for 30-40 minutes until onions are deeply browned and jammy. Add balsamic vinegar to deglaze, then remove from heat to cool.
Assemble and Bake
  1. Preheat oven to 400°F (200°C). Lay pastry on parchment. Score a 1-inch border and dock the center with a fork.
  2. Spread onions inside the border, top with Gruyère and thyme. Brush the border with egg wash.
  3. Bake for 20-25 minutes until the border is golden-brown and the cheese is bubbly and toasted.

Notes

Use a heavy pan for onions to prevent burning.
Ensure the puff pastry is very cold when it goes into the oven for maximum rise.
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