Preheat Oven: Preheat your oven to 375°F (190°C).

Blind Bake the Crust: Place the pie crust in a 9-inch pie dish and crimp the edges. Line the crust with parchment paper, fill it with pie weights or dried beans, and bake for about 10 minutes. Take out the weights and return the dish to the oven for another 5 minutes, or until it turns a light golden color. Let it cool before setting aside.

Cook the Bacon and Shallot: In a skillet over medium heat, cook the bacon pieces until they are crispy, about 6-8 minutes. Transfer the bacon to a paper towel-lined plate to drain, then pour off most of the bacon fat from the skillet, leaving roughly 1 tablespoon behind. Add the minced shallot and red pepper flakes (if using) to the skillet and sauté for 2-3 minutes, until softened. Remove from heat.

Prepare the Egg Mixture: In a medium mixing bowl, whisk together the eggs, half-and-half (or milk and cream mixture), salt, black pepper, nutmeg, and cayenne pepper. Make sure the mixture is smooth and well-combined.

Assemble the Quiche: Sprinkle the cooked bacon, sautéed shallot, grated Gruyère, and Parmesan cheese evenly over the bottom of the pre-baked pie crust. Pour the egg mixture over the bacon and cheese, ensuring even coverage.

Bake the Quiche: Place the quiche on a baking sheet and bake for 35-40 minutes, or until the center is just set and a knife inserted comes out clean. The quiche should have a slight jiggle in the center but will firm up as it cools.

Garnish and Serve: Garnish with fresh parsley leaves and serve warm or at room temperature.