Caramelized Onion Gruyere Tart (Print version)

Flaky tart featuring slow-cooked onions and creamy Gruyere in a buttery pastry, ideal for brunch or light lunch.

# What You Need:

→ Pastry

01 - 1 sheet (9-inch) thawed puff pastry

→ 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

→ 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:

01 - Preheat oven to 375°F. Roll out puff pastry and fit into 9-inch tart pan with removable bottom. Trim excess and prick base with fork. Refrigerate while preparing filling.
02 - In 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 golden brown and deeply caramelized. Add thyme during final 5 minutes. Cool slightly.
03 - In mixing bowl, whisk together eggs, heavy cream, milk, nutmeg, salt, and pepper until smooth and fully combined.
04 - Brush chilled pastry base with Dijon mustard. Spread caramelized onions evenly over base. Sprinkle grated Gruyere cheese over onions. Pour egg custard mixture evenly over cheese and onions.
05 - Bake for 35-40 minutes until filling is set and top is golden brown. Remove from oven and cool for 10 minutes before slicing and serving.

# Expert tips:

01 -
  • The caramelized onions taste like they've been kissed by time itself—sweet, deep, and nothing like raw onion.
  • It looks restaurant-worthy but actually comes together on a quiet afternoon with minimal fuss.
  • Works equally well warm, room temperature, or even chilled the next day, making it endlessly flexible for your schedule.
02 -
  • Don't rush the onion caramelization; I learned this by trying to speed it up once, and they just turned burnt and bitter instead of sweet.
  • The custard will look slightly underbaked when you pull it out, but it continues to cook as it cools—trust the 10-minute rest period.
03 -
  • Grate your Gruyere fresh if possible—pre-shredded cheese contains anti-caking agents that can make the filling slightly grainy.
  • If your pastry starts browning too quickly around the edges, drape a piece of foil loosely over the tart during the final 15 minutes of baking.
Go back