Heat oven to 350°F. Spray 8-inch square (2-quart) glass baking dish with cooking spray. Cut any large meatballs in half. Cook and drain pasta as directed on package.
Meanwhile, in 2-quart saucepan, mix milk and sauce mix with wire whisk. Heat to boiling over medium heat, stirring constantly. Boil 1 minute, stirring constantly. Stir in cheeses until melted. Stir in pasta and meatballs. Spoon into baking dish. In small bowl, mix butter, bread crumbs and onions; sprinkle over top.
Bake uncovered 30 to 35 minutes or until bubbly and top is golden brown.