Heat oven to 400F. In lrg. skillet over med.-high heat, brown ground beef, onions and bell pepper. Drain. Stir in pizza sauce, pepperoni, Italian seasoning and mushrooms. Bring to a boil. REduce heat to low; simmer uncovered 10 mins., stirring occasionally. Meanwhile, in sm. bowl combine milk, oil and eggs; beat 1 min. at med. speed. Lightly spoon flour into measuring cup; level off. Add flour and salt; beat 2 mins. at med. speed or until smooth. Spoon meat mixture evenly into ungreased 13x9" baking dish. Place cheese slices over hot meat mixture. Pour topping evenly over cheese, covering complete; sprinkle with Parmesan cheese. Bake at 400F for 20-30 mins. or until puffed and deep golden brown. Makes 6-8 servings. "It's fun to watch the 'crust' rise while this unique casserole is baking. The secret to the tender crust is following the recipe directions carefully and using the pan size called for."