1 unbaked crescent sheet
8 slices bacon, cut into 1 inch pieces
1/2 cup chopped leeks (white and pale green parts only)
1/2 cup chopped shallots
kosher salt and freshly ground black pepper to taste
1 pinch cayenne pepper, or more to taste
3 large eggs
1 cup heavy cream
3/4 cup milk
1 teaspoon chopped fresh thyme
16 ounces shredded Gruyere cheese
Preheat oven to 425 degrees F (220 degrees C).
Roll pie dough to fit a 9×13 inch greased baking dish. Bake crescent in the preheated oven for 10-11 minutes. Bake until golden brown.
Reduce oven to 325 degrees F (165 degrees C).
Cook bacon in a skillet over medium heat until browned and cooked through, 8 to 10 minutes. Remove from pan to drain. Blot out some of the oil from the skillet, leaving 1 to 2 teaspoons. Add leeks, onion, salt, black pepper, and cayenne pepper. Cook and stir until tender and golden brown, 5 to 7 minutes. Set aside.
Whisk eggs, egg yolks, cream, and milk together in a large bowl. Add thyme and stir to combine.
Sprinkle 1/2 of onion-leek mixture on the bottom of the baked crust. Top with 1/3 cooked bacon and 1/2 Gruyere cheese. Ladle in egg mixture carefully. Sprinkle with remaining onion-leek mixture, remaining bacon, and remaining Gruyere cheese.
Bake filled quiche in the preheated oven until browned and set, and no longer jiggly in the center, 40 to 45 minutes. Allow to cool slightly before serving.