Caramelized Onion Gruyere Tart (Printable)

Savory tart combining sweet caramelized onions and rich Gruyere cheese in a flaky crust.

# What You Need:

→ Pastry

01 - 1 9-inch round sheet puff pastry, thawed

→ 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 granulated 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 9-inch tart pan with removable bottom. Trim excess pastry and prick base with fork. Refrigerate while preparing filling.
02 - In large skillet over medium-low heat, melt butter with olive oil. Add onions, salt, pepper, and sugar. Cook stirring frequently for 30-35 minutes until golden brown and caramelized. Stir in thyme during final 5 minutes. Remove from heat and cool slightly.
03 - In mixing bowl, whisk together eggs, heavy cream, milk, nutmeg, salt, and pepper until smooth and combined.
04 - Brush chilled pastry base with Dijon mustard. Spread caramelized onions evenly over base. Sprinkle grated Gruyere cheese over onions. Pour egg mixture evenly over onions and cheese.
05 - Bake for 35-40 minutes until filling is set and top is golden brown. Cool for 10 minutes before slicing and serving.

# Expert Suggestions:

01 -
  • The caramelized onions taste like dessert meets dinner, naturally sweet without a grain of added sugar beyond a teaspoon.
  • It's fancier than it looks, meaning you can serve it to people who matter without spending all day in the kitchen.
  • Leftovers actually improve overnight as the flavors settle and deepen, making it perfect for meal prep.
02 -
  • Caramelizing onions truly takes 30 to 35 minutes on medium-low heat; impatience is the number one reason people end up with cooked rather than caramelized onions, which tastes completely different.
  • Don't skip pricking the pastry base and don't skip the mustard layer; these two steps are the difference between crispy, golden pastry and a soggy bottom that haunts you.
03 -
  • A splash of white wine stirred into the onions during the final minutes adds an almost imperceptible depth that makes people wonder what your secret ingredient is.
  • Room temperature eggs whisk more smoothly into the cream, creating a silkier custard, though cold eggs from the fridge work just fine if you're short on time.
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