# Components:
→ 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
# Method:
01 - Preheat oven to 375°F. Roll out puff pastry and fit into 9-inch tart pan with removable bottom, trimming excess. Prick base with fork and 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, 30-35 minutes until golden 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 cheese and onions.
05 - Bake 35-40 minutes until filling is set and top is golden brown. Cool for 10 minutes before slicing and serving.
