A savory tart with buttery flaky crust, sweet caramelized onions, and melted Gruyere cheese for an elegant meal.
# 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 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
# Step-by-Step:
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 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 to 35 minutes until onions are golden brown and caramelized. Stir in thyme during the last 5 minutes and remove from heat to 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 chilled pastry shell with Dijon mustard. Spread caramelized onions evenly over the base. Sprinkle grated Gruyere cheese over the onions. Pour custard mixture evenly over the cheese and onions.
05 - Bake for 35 to 40 minutes until the filling is set and the top is golden brown.
06 - Allow tart to cool for 10 minutes before slicing and serving.
