Cook and stir sausage in a large skillet over medium heat until browned, 5 to 8 minutes. Drain.
Mix Parmesan cheese, mozzarella cheese, ricotta cheese, and eggs together in a bowl. Stir in sausage, prosciutto, and parsley.
Roll 1 pie crust into an 11-inch circle on a lightly floured work surface. Transfer to a pie plate, leaving a 1/2-inch overhang. Pierce crust all over with a fork. Spread cheese and sausage mixture over crust. Roll second pie crust into a 12-inch circle; place on top. Seal and flute edges. Cut several slits in the top crust to permit steam to escape.
Bake in the preheated oven until center is set and crust is browned, about 1 hour.