For crust,in a large bowl, beat the mashed potatoes, sour cream, mozzarella cheese, egg, salt and pepper until blended.
Spread potato mixture onto a 12-in. pizza pan. Build up edges slightly. Sprinkle with bread crumbs. Bake at 350° for 20 minutes.
Meanwhile, for topping, in a large skillet, cook beef, luncheon meat and onion over medium heat until meat is no longer pink: drain Stir in pizza sauce; set aside.
Remove crust from oven; increase temperature to 400°. Spoon topping mixture evenly over crust to within 1/2-in. of edges. Sprinkle with mozzarella and Parmesan cheeses. Bake another 10 minutes or until cheese is melted.