For crust: Let piecrust stand at room temperature according to package directions. Line a 9-inch pie plate with the pastry. Crimp edge as desired. Line unpricked pastry shell with a double thickness of foil. Bake in a 450 degree F oven for 8 minutes. Remove foil. Bake for 4 to 5 minutes more or until pastry is set and dry. Reduce oven temperature to 325 degree F.
Meanwhile, in a large bowl, stir together the thawed souffle, milk, eggs, and sour cream; set aside. In a medium skillet, cook sausage, mushrooms and onion until meat is no longer pink; drain. Add sausage mixture to souffle mixture; stir in cheeses.
Pour egg mixture into hot, baked pastry shell. Bake in the 325 degree F oven for 45 to 50 minutes or until a knife inserted near center comes out clean. If necessary, cover edge of crust with foil to prevent overbrowning. Let stand for 15 minutes before serving. Garnish each serving with spinach leaves, if you like. Makes 6 servings.