# What You Need:
→ Pastry
01 - 1 sheet (9-inch round) 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
# How-To:
01 - Set oven temperature to 375°F and allow to fully preheat.
02 - Roll out puff pastry and fit into a 9-inch tart pan with removable bottom. Trim excess pastry from edges. Pierce base with fork to prevent puffing. Refrigerate while preparing remaining components.
03 - Heat butter and olive oil in large skillet over medium-low heat. Add sliced onions, salt, pepper, and sugar. Cook while stirring frequently for 30 to 35 minutes until onions achieve deep golden-brown color and sweet caramelized flavor. Add thyme leaves during final 5 minutes. Remove from heat and cool slightly.
04 - In mixing bowl, whisk together eggs, heavy cream, whole milk, nutmeg, salt, and black pepper until completely smooth.
05 - Brush mustard evenly across base of chilled pastry shell. Distribute caramelized onions in single layer over mustard. Sprinkle grated Gruyere cheese evenly over onions. Pour custard mixture slowly over cheese and onions, distributing uniformly.
06 - Place in preheated oven and bake for 35 to 40 minutes until custard filling is set and top surface achieves light golden-brown color.
07 - Remove from oven and allow to cool for 10 minutes before slicing. Transfer slices to serving plates while warm.
