COOK Macaroni in large saucepan as directed on package. Remove from pan; drain.
MELT margarine in same pan on medium heat. Add onions; cook and stir 2 min. Stir in contents of Seasoning packet, Cheese Sauce Mix and milk. Add macaroni; mix lightly. Stir in sour cream.
POUR into 1-1/2-qt. casserole sprayed with cooking spray. Top with shredded cheese and Crumb Topping.
BAKE 10 min. or until macaroni mixture is heated through and shredded cheese is melted.