HEAT oven to 375ºF.
COOK macaroni as directed on package, omitting the salt. Meanwhile, brown meat with peppers and onions in large nonstick skillet. Stir in Velveeta and pizza sauce; cook and stir 5 min. or until meat mixture is hot and bubbly and Velveeta is completely melted.
SPOON macaroni into 8-inch square baking dish; top with meat mixture and mozzarella. Cover.
BAKE 10 min. or until casserole is heated through and mozzarella cheese is melted.