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A golden-brown onion tart with caramelized onions on top.

Classic Gruyère and Thyme Onion Tart

This classic tart features a buttery, flaky crust filled with sweet, slow-caramelized onions and a rich, savory Gruyère cheese custard. Perfect for an appetizer or light meal, it's a comforting dish with elegant flavors.
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Prep Time 45 minutes
Cook Time 1 hour 30 minutes
Total Time 2 hours 15 minutes
Servings 8 slices
Calories 350 kcal

Equipment

  • large bowl
  • pastry blender
  • plastic wrap
  • rolling pin
  • 9-inch tart pan
  • parchment paper
  • pie weights or dried beans
  • large skillet or Dutch oven
  • whisk
  • wire rack

Ingredients
  

For the Pastry:

  • 1 ¼ cups all-purpose flour
  • ½ teaspoon salt
  • 8 tablespoons unsalted butter, very cold and cubed
  • 3-4 tablespoons ice water

For the Filling:

  • 3 large yellow onions about 2 lb
  • 2 tablespoons unsalted butter
  • 1 tablespoon olive oil
  • ½ teaspoon salt plus more to taste
  • ¼ teaspoon freshly ground black pepper plus more to taste
  • nutmeg Pinch
  • 3 large eggs
  • 1 cup heavy cream
  • 1 cup grated Gruyère cheese divided
  • 1 tablespoon fresh thyme leaves, chopped optional, for garnish

Instructions
 

Prepare the Pastry

  • Whisk together flour and salt, then cut in cold butter until crumbly.
  • Gradually add ice water until the dough forms, then refrigerate for at least 30 minutes.

Roll out the Pastry

  • Roll dough to an 11-12 inch round and transfer to a 9-inch tart pan with a removable bottom.
  • Press into the pan, trim excess, prick the bottom with a fork, and chill for 20 minutes.

Blind Bake the Crust

  • Preheat oven to 375°F (190°C), then line the pastry shell with parchment and pie weights.
  • Bake for 15 minutes, remove weights and parchment, then bake for another 5-7 minutes until golden.
  • Cool slightly and reduce oven temperature to 350°F (175°C).

Caramelize the Onions

  • Melt butter and olive oil in a large skillet over medium-low heat, then add sliced onions and salt.
  • Cover and cook for 10-15 minutes until softened, then uncover and cook for 25-35 minutes until deep golden brown.
  • Let cool slightly.

Make the Custard

  • Whisk together eggs, heavy cream, salt, pepper, and nutmeg in a medium bowl.
  • Stir in ¾ cup of Gruyère cheese, reserving the rest for topping.

Assemble the Tart

  • Spread cooled caramelized onions evenly over the blind-baked pastry shell.
  • Pour the custard mixture over the onions, then sprinkle with the remaining Gruyère cheese.

Bake the Tart

  • Bake at 350°F (175°C) for 35-45 minutes, or until the custard is set and golden brown.
  • If browning too quickly, loosely tent with aluminum foil.

Cool and Serve

  • Remove from oven and cool on a wire rack for 15-20 minutes before slicing.
  • Garnish with fresh thyme leaves if desired.

Notes

For the best flavor, take your time caramelizing the onions; it's the key to this tart. Ensure your butter is very cold for a flaky pastry. Allowing the tart to cool completely is crucial for the custard to set properly, making for clean slices. This tart can be served warm or at room temperature. Leftovers can be stored in the refrigerator for up to 3 days and reheated gently.