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

Classic Creamy Onion Tart

A savory tart featuring sweet caramelized onions baked in a creamy, rich Gruyere custard and a flaky pastry crust. Perfect for a sophisticated appetizer or light meal.
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Prep Time 45 minutes
Cook Time 1 hour 30 minutes
Total Time 2 hours 15 minutes
Servings 6 slices
Calories 350 kcal

Equipment

  • Large Bowl
  • Pastry Blender
  • Plastic Wrap
  • 9-inch Tart Pan with Removable Bottom
  • Parchment Paper
  • Pie Weights or Dried Beans
  • Large Skillet or Dutch Oven
  • Medium Bowl

Ingredients
  

For the Pastry (or 1 pre-made pie crust):

  • 1 ½ cups all-purpose flour
  • ½ teaspoon salt
  • ½ cup cold unsalted butter cut into small cubes
  • 4-6 tablespoons ice water

For the Caramelized Onions:

  • 3 large yellow onions thinly sliced
  • 2 tablespoons olive oil
  • 1 tablespoon butter
  • ½ teaspoon salt
  • ¼ teaspoon black pepper
  • 1 teaspoon sugar optional, to aid caramelization

For the Creamy & Rich Filling:

  • 2 large eggs
  • ¾ cup heavy cream
  • ½ cup shredded Gruyere cheese or Swiss
  • nutmeg Pinch of

Instructions
 

Prepare the Pastry

  • If making pastry from scratch, combine flour and salt, then cut in cold butter until coarse crumbs form. Gradually add ice water, mix until combined, form into a disc, wrap, and chill for at least 30 minutes; otherwise, use a pre-made crust.

Blind Bake the Crust

  • Preheat oven to 375°F (190°C), then roll out the pastry into a 9-inch tart pan with a removable bottom and trim edges. Line with parchment paper, fill with pie weights, bake for 15 minutes, then remove weights and paper and bake for another 5-10 minutes.

Caramelize the Onions

  • Melt olive oil and butter in a large skillet over medium-low heat, then add sliced onions, salt, pepper, and optional sugar. Cook slowly for 30-45 minutes, stirring occasionally, until deeply golden brown and very soft to achieve sweetness.

Prepare the Filling

  • In a medium bowl, whisk together eggs, heavy cream, Gruyere cheese, and nutmeg until well combined.

Assemble the Tart

  • Evenly spread the caramelized onions into the prepared tart crust, then carefully pour the creamy filling over the onions.

Bake

  • Bake the tart for 30-40 minutes, or until the filling is set and golden brown, and a knife inserted near the center comes out clean.

Cool and Serve

  • Allow the tart to cool for at least 15-20 minutes before slicing to ensure the filling sets completely.

Notes

Caramelizing the onions slowly is crucial for developing their sweet flavor. To ensure a crispy crust, make sure to blind bake it thoroughly. The tart is delicious served warm or at room temperature. For variations, consider adding a sprinkle of fresh thyme or a different cheese like Parmesan.