Preparation
Cook macaroni according to package directions. Meanwhile, in a saucepan, sauté onion in ½ cup butter. Add flour; whisk until blended. Gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in cheese, mustard, salt and pepper until smooth.
Drain macaroni and add to sauce; stir until coated. Transfer to a greased 13-in. x 9-in. x 2-in. baking dish. Melt the remaining butter; toss with bread crumbs. Sprinkle over macaroni mixture. Bake, uncovered, at 350° for 30-35 minutes or until heated through.