Preheat oven to 375°F. Cook macaroni al dente according to package directions. Drain. Pour into buttered shallow 2-quart baking dish.
In saucepan, melt 3 tablespoons butter; whisk in flour, salt and pepper. Slowly whisk in milk; cook and stir until thickened. Add onion, cheese, dry mustard and Worcestershire sauce. Cook and stir until cheese melts. Pour mixture over macaroni and mix lightly. Melt remaining butter in small skillet; add bread crumbs. Stir, coating crumbs. Sprinkle over macaroni. Bake 25 minutes or until top is browned.