Cook macaroni according to package directions; drain and set aside.
Meanwhile, melt 1/4 cup butter in a large heavy saucepan over low heat; whisk in flour until smooth. Cook one minute, whisking constantly. Gradually whisk in milk. Cook over medium heat, whisking constantly, until mixture is thickened and bubbly. Reduce heat and add cheese, onion, salt, Worcestershire sauce, pepper and mustard; stir until cheese melts. Add macaroni, and mix well.
Divide mixture evenly into 6 (8-ounce) greased ramekins, or a greased 2-quart baking dish. Sprinkle with bread crumbs and dot lightly with additional butter. Bake, uncovered, at 375 degrees for 20 to 30 minutes.