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

Classic 8-Star Onion Tart

This classic 8-star onion tart features a flaky pastry crust filled with sweet caramelized onions and a rich, savory Gruyère cheese custard. Perfect for a sophisticated appetizer or a light meal.
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Prep Time 40 minutes
Cook Time 2 hours 10 minutes
Total Time 2 hours 50 minutes
Servings 8 slices
Calories 400 kcal

Equipment

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

Ingredients
  

For the Pastry Crust:

  • 1 ½ cups all-purpose flour
  • ½ teaspoon salt
  • ½ cup unsalted butter very cold and cut into small cubes
  • ¼ cup ice water or more as needed

For the Filling:

  • 4 large yellow onions about 3 lbs, thinly sliced
  • 2 tablespoons unsalted butter
  • 1 tablespoon olive oil
  • ½ teaspoon salt
  • ¼ teaspoon black pepper
  • 1 sprig fresh thyme optional

For the Custard:

  • 2 large eggs
  • 1 cup heavy cream
  • ½ cup whole milk
  • ¼ teaspoon salt
  • ⅛ teaspoon black pepper
  • Pinch freshly grated nutmeg
  • ½ cup Gruyère cheese grated

Instructions
 

Pastry Dough

  • Whisk flour and salt, then cut in cold butter until crumbly. Gradually add ice water and mix until a dough forms. Refrigerate for at least 30 minutes after shaping into a disc.

Blind Bake the Crust

  • Preheat oven to 375°F (190°C). Roll out dough and transfer to a 9-inch tart pan with a removable bottom. Line with parchment paper and pie weights, then bake for 15 minutes, remove weights, and bake for another 5-7 minutes until lightly golden. Reduce oven to 350°F (175°C).

Caramelize the Onions

  • Melt butter and olive oil in a large skillet over medium-low heat. Add sliced onions, salt, pepper, and thyme, then cook slowly for 45-60 minutes until deeply golden and sweet. Remove thyme and cool slightly.

Prepare the Custard

  • Whisk together eggs, heavy cream, milk, salt, pepper, and nutmeg in a medium bowl until well combined.

Assemble the Tart

  • Spread cooled caramelized onions evenly over the blind-baked crust, then sprinkle with grated Gruyère cheese. Carefully pour the custard mixture over the cheese and onions.

Bake the Tart

  • Bake for 35-45 minutes until the custard is set and the top is golden brown.

Cool and Serve

  • Allow the tart to cool on a wire rack for at least 15-20 minutes before slicing and serving.

Notes

For an extra touch of flavor, you can brush the baked crust with a little egg white before filling to create a moisture barrier.