Caramelized Onion Gruyere Tart (Printable)

Flaky tart featuring caramelized onions and Gruyere cheese for a savory, elegant dish.

# What You'll Need:

→ 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

# How To Make It:

01 - Preheat oven to 375°F. Roll out puff pastry and fit into a 9-inch tart pan with removable bottom, trimming excess. Prick base with a fork and refrigerate while preparing filling.
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 30-35 minutes until onions are deep golden brown and caramelized. Stir in thyme during the final 5 minutes, then remove from heat and cool slightly.
03 - In a mixing bowl, whisk together eggs, heavy cream, milk, nutmeg, salt, and pepper until smooth and well combined.
04 - Brush the base of the chilled pastry shell with Dijon mustard. Spread caramelized onions evenly over the base, then sprinkle with grated Gruyere cheese. Pour custard mixture evenly over onions and cheese.
05 - Bake for 35-40 minutes until the filling is set and the top is golden brown.
06 - Remove from oven and allow to cool for 10 minutes before slicing and serving.

# Expert Suggestions:

01 -
  • The pastry gets impossibly flaky and golden while the filling stays silky and rich, like tasting two textures at once.
  • Those onions take patience, but the reward is a depth of flavor that tastes like you've been cooking all day, even though the actual hands-on time is minimal.
  • It looks elegant enough for a dinner party but feels casual enough to eat in your kitchen with coffee on a random Wednesday.
02 -
  • Rushing the onions is the biggest mistake—low heat and frequent stirring are what turn them sweet and caramelized, and there's genuinely no shortcut that works as well.
  • Don't skip pricking the pastry base; I learned this the hard way when the bottom puffed up and made the tart impossible to slice cleanly.
03 -
  • If your pastry starts browning too fast before the filling sets, tent it loosely with foil—it won't stop the filling from cooking but protects the edges from getting dark.
  • Let the tart rest those full 10 minutes before slicing; it sounds simple, but it really does make the difference between a tart that holds together and one that falls apart on the plate.
Go Back