Preheat the oven to 350 degrees.

In a large skillet, melt 2 Tablespoons of the butter over medium heat. Add the mushrooms and cook, undisturbed, for a few minutes, or until the bottom side is golden. Flip and continue to cook until mushrooms are golden on both sides. Season with salt. Remove to plate.

In the same skillet, melt the remaining butter. Add the leeks and saute until softened, about 5-7 minutes. Remove the leeks to a plate.

In a large bowl, whisk together the eggs, milk, cream, salt, and flour.

Sprinkle some of the gruyere cheese on the bottom of the pie crust. Pour half the egg mixture in followed by half of the gruyere cheese and half of the mushrooms and leeks. Pour the remaining egg mixture on top along with the rest of the mushrooms, leeks, cheese, and gruyere. Top with Parmesan cheese.

Bake the quiche in the preheated oven for 60-80 minutes or until very golden brown and set in the center. If the pie crust gets too brown, shield the outside crust with tin foil. The quiche can be served hot or at room temperature and should keep for several days in the refrigerator. If you'd like to make this ahead of time, allow the quiche to cool, then refrigerate overnight. When ready to serve, heat the oven to 350 degrees. Slice the quiche and put the slices on a baking sheet. Heat for about 10 minutes or until hot throughout.