In a bowl, combine potatoes, egg, Parmesan cheese, butter and salt. Press onto bottom and up the sides of a greased 10-in. pie plate. Spread pizza sauce over potato mixture to within 1/2 in. of edges. Top with pepperoni and mushrooms. Sprinkle with mozzarella cheese. Bake at 400° for 25-30 minutes or until heated through and cheese is melted.
Editor's Note: An 11x7x2-in. baking dish can be used instead of the pie plate.