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Caramelized Onion and Gruyere Puff Pastry Tartlets

Caramelized onion and Gruyere puff pastry tartlets bake into flaky, golden squares with sweet jammy onions and finely grated cheese. They’re assembled on a baking sheet and baked until the bottoms are crisp.
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Prep Time 20 minutes
Cook Time 1 hour 4 minutes
Cooling and resting 15 minutes
Total Time 1 hour 39 minutes
Servings: 9 servings
Course: Appetizer
Cuisine: French
Calories: 300
Ingredients Equipment Method Notes

Ingredients
  

Caramelized onions
  • 2 tbsp unsalted butter
  • 1 tbsp olive oil
  • 2 onions large yellow, about 1 1/2 pounds, thinly sliced
  • 0.5 tsp kosher salt
  • 0.25 tsp black pepper
  • 1 tsp thyme fresh leaves
Tartlets
  • 1 puff pastry sheet frozen, from a 17.3-ounce package, thawed but cold
  • 4 oz Gruyere cheese finely grated
  • 1 egg large
  • 1 tbsp water

Equipment

  • 1 12-inch skillet
  • 1 18x13-inch rimmed baking sheet

Method
 

Caramelize the onions
  1. Melt the butter and olive oil in a 12-inch skillet over medium heat.
  2. Add the onions and salt and cook, stirring frequently, for 8 to 10 minutes, until softened.
  3. Reduce the heat to medium-low and cook the onions for 30 to 40 minutes, stirring frequently, until deeply golden and jammy.
  4. Stir in the black pepper and thyme for the final 2 minutes.
  5. Transfer the onions to a plate and let them cool for 10 minutes.
Assemble and bake
  1. Heat the oven to 400°F.
  2. Line an 18x13-inch rimmed baking sheet with parchment paper.
  3. Unfold the puff pastry on a cutting board and cut it into 9 equal squares.
  4. Transfer the squares to the prepared baking sheet, leaving space between them.
  5. Score a 1/2-inch border on each square without cutting through.
  6. Prick the centers several times with a fork.
  7. Divide the Gruyere among the pastry centers.
  8. Top the pastry centers with the cooled onions.
  9. Whisk the egg and water together in a small bowl.
  10. Brush the pastry borders with the egg wash.
  11. Bake for 18 to 22 minutes, until the pastry is puffed and deep golden and the bottoms are crisp.
  12. Let the tartlets rest on the baking sheet for 5 minutes before serving.

Notes

For the best texture, serve the tartlets soon after baking. Store leftovers in an airtight container in the refrigerator for up to 3 days; freezing is not recommended. For a vegetarian version, use Gruyere made with vegetarian rennet.