Pre heat the oven to 425°.
Place your pie pans onto a baking sheet. Carefully roll out your pie crusts into circles slightly larger than the pie pans. Gently transfer the pie crust into the pans.
Line the pie shell with parchment paper and fill with dried beans or pie weights. (This prevents the crust from puffing too much.)
Bake your pie crust for 15 minutes, then carefully remove the beans/weights, and finish baking till the base is firm, about 10 more minutes.
While your pie crust is baking prepare your filling. When your crust is done, remove it from the oven and lower the temperature to 325°.
Put 3 tablespoons of butter or olive oil into a deep skillet over medium heat, then add the leeks and cook, stirring frequently for a few minutes. Add the mushrooms and cook about 5 minutes more.
Layer this leek-mushroom mixture into the pie shell. Add the chopped parsley to the mixture and set aside.
Whisk the eggs and milk in a bowl and then pour over the leek- mushroom mixture.
Carefully place the quiche into the preheated oven and bake 30- 40 minutes, until the filling is almost firm but still a little jiggly. Let cool for at least 15 minutes before slicing. Serve warm or at room temperature