Stir together flour, Parmesan cheese garlic salt and seasoning. Set aside. In large mixing bowl, beat egg whites with cream of tartar at high speed until stiff but not dry, just until whites no longer slip when bowl is tilted.
Gently, but thoroughly, fold in reserved flour mixture. Spread evenly in greased 12-inch diameter pizza pan or 13 x 9 x 2-inch baking pan. Bake in preheated 375°F oven until firm and delicately browned at edges about 8 to 10 minutes.
Remove from oven. Spread with tomato sauce. Top with mushroom slices, pepper rings and mozzarella cheese. Return to oven and continue baking until cheese is melted and lightly browned, about 7 to 10 minutes. To serve, cut into wedges or squares.