Caramelized Onion Gruyere Tart (Print)

Flaky tart filled with sweet caramelized onions and savory Gruyere, ideal for elegant brunch gatherings.

# Components:

→ Pastry

01 - 1 sheet (9-inch round) 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

# Directions:

01 - Preheat oven to 375°F
02 - Roll out puff pastry and fit into a 9-inch tart pan with removable bottom. Trim excess pastry. Prick base with fork. Chill in refrigerator while preparing remaining components.
03 - Melt butter with olive oil in large skillet over medium-low heat. Add onions, salt, pepper, and sugar. Cook while stirring frequently for 30 to 35 minutes until onions turn golden brown and caramelized. Add thyme during final 5 minutes. Remove from heat and cool slightly.
04 - Whisk together eggs, heavy cream, milk, nutmeg, salt, and pepper in mixing bowl until smooth.
05 - Brush inside of chilled pastry shell with Dijon mustard. Spread caramelized onions evenly over base. Sprinkle grated Gruyere cheese over onions. Pour custard mixture evenly over cheese and onions.
06 - Bake for 35 to 40 minutes until filling is set and top is golden brown.
07 - Cool for 10 minutes before slicing and serving.

# Expert Advice:

01 -
  • Those caramelized onions taste like pure umami magic, and people will genuinely ask for your secret.
  • It's elegant enough to impress at brunch but honestly simple enough that you can make it on a random Tuesday and feel accomplished.
  • The creamy custard filling transforms humble ingredients into something that tastes like you spent all day in the kitchen.
02 -
  • Those 30 to 35 minutes of caramelizing are absolutely essential—if you rush it or crank up the heat, you'll end up with cooked onions instead of caramelized ones, and the whole tart suffers.
  • Don't skip the Dijon mustard step; it's a flavor bridge that prevents the pastry from getting soggy and adds a subtle sophistication that ties everything together.
03 -
  • If you want to add depth, a splash of white wine swirled into the onions during the last few minutes of cooking creates an almost French restaurant vibe—let it reduce completely so there's no alcohol taste, just richness.
  • Make this tart a few hours ahead and reheat it gently in a 300°F oven for 10 minutes before serving; the flavors actually deepen and marry together overnight in the fridge.
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