Caramelized Onion Gruyere Tart (Printable Version)

Flaky pastry filled with golden caramelized onions and creamy Gruyere cheese for an elegant meal.

# Components:

→ Pastry

01 - 1 9-inch round sheet of puff pastry, thawed if frozen

→ Caramelized Onions

02 - 2 tablespoons unsalted butter
03 - 2 tablespoons olive oil
04 - 4 large yellow onions, thinly sliced
05 - 1 teaspoon salt
06 - 1/2 teaspoon freshly ground black pepper
07 - 1 teaspoon sugar
08 - 2 teaspoons fresh thyme leaves or 1 teaspoon dried thyme

→ Filling

09 - 3 large eggs
10 - 1 cup heavy cream
11 - 1/2 cup whole milk
12 - 1/4 teaspoon ground nutmeg
13 - 1/2 teaspoon salt
14 - 1/4 teaspoon freshly ground black pepper
15 - 1 1/2 cups grated Gruyere cheese
16 - 1 tablespoon Dijon mustard

# Directions:

01 - Preheat oven to 375°F. Roll out puff pastry and fit into a 9-inch tart pan with removable bottom. Trim excess pastry and prick the base with a fork. Refrigerate while preparing remaining components.
02 - In a large skillet over medium-low heat, melt butter with olive oil. Add sliced onions, salt, pepper, and sugar. Cook, stirring frequently, for 30 to 35 minutes until onions develop deep golden color and sweetness. Stir in thyme during final 5 minutes. Remove from heat and cool slightly.
03 - In a mixing bowl, whisk together eggs, heavy cream, milk, nutmeg, salt, and pepper until thoroughly combined and smooth.
04 - Brush chilled pastry base with Dijon mustard. Spread caramelized onions evenly across the base. Sprinkle grated Gruyere cheese over onions. Pour custard mixture evenly over cheese and onions.
05 - Bake for 35 to 40 minutes until filling is set and top is golden brown. Remove from oven and let cool for 10 minutes before slicing.

# Expert Tips:

01 -
  • The caramelized onions taste like pure umami magic—deeply savory in a way that makes people ask what your secret is.
  • It looks fancy enough to serve guests but honest enough to eat alone with a glass of wine on a weeknight.
  • The whole thing comes together in just over an hour, which means you can actually pull off an elegant brunch without stress.
02 -
  • Do not rush the caramelization process—I learned this the hard way by turning the heat up high and ending up with burnt onions that tasted bitter and made the whole tart unpleasant.
  • If your custard looks like it's setting too fast around the edges while the center is still very loose, tent the tart loosely with foil to slow the browning and give the middle more time to cook through.
03 -
  • Add a splash of dry white wine to the onions during the last 10 minutes of caramelization for added depth and a subtle sweetness that elevates the whole dish.
  • Grate your Gruyere fresh from the block instead of using pre-shredded—it melts more smoothly and creates a creamier filling without the anti-caking agents that can make things grainy.
Return