Cook spaghetti as directions on package; drain well and put back into pan.
In a separate bowl, beat milk and egg together and mix into spaghetti. Add seasoned salt and garlic salt. Pour into a greased 9 x 13-inch pan. Spread spaghetti sauce over top of spaghetti; stir to mix. Sprinkle 2 cups of the mozzarella cheese over sauce/spaghetti mixture. Add pepperoni or ground beef or Italian sausage, mushrooms, green bell pepper and onion to top of spaghetti mixture; sprinkle remaining 2 cups mozzarella cheese over top of casserole and bake, uncovered, at 300 degrees F for 1 hour.
Serves 6 to 8.