Caramelized Onion Gruyere Tart (Printable Version)

Savory tart with caramelized onions and Gruyere cheese in a buttery, flaky crust.

# 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

# Directions:

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

# Additional Tips::

01 -
  • The slow caramelization transforms ordinary onions into something so deeply sweet and complex that even skeptics will ask for seconds.
  • It looks restaurant-worthy but requires almost no skill, just patience and a good skillet.
  • Works equally well for an elegant brunch, a casual lunch, or that moment when you need to feed people something that feels special.
02 -
  • Don't skip the mustard brushing on the pastry base; it's the invisible barrier that prevents the bottom from becoming soggy while the filling sets.
  • If your onions aren't turning golden after twenty-five minutes, increase the heat slightly—they should be actively caramelizing, not just stewing.
03 -
  • A splash of white wine added to the onions in the final few minutes deepens their flavor with a subtle acidity that makes them taste even more sophisticated.
  • Grate your Gruyere fresh rather than using pre-shredded, as the anticaking agents in packaged cheese can prevent the custard from setting properly.
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