How to Make It
Preheat oven to 350°.
Cook macaroni according to package directions. Drain and set aside. Melt butter in a large heavy saucepan over low heat; add flour, stirring until smooth. Cook 3 to 5 minutes, stirring constantly. Turn heat to medium; gradually whisk in milk, and cook over medium heat, stirring or whisking constantly until thickened, about 10 minutes. Stir in pasta, mustard, and next 4 ingredients, stirring just until cheese begins to melt.
Pour pasta mixture into a lightly greased 13- x 9-inch baking dish. Sprinkle with Parmigiano-Reggiano or more Cheddar. If desired, top with fresh breadcrumbs, and drizzle evenly with melted butter.
Bake, uncovered, at 350° for 25 minutes or until bubbly and golden. Let stand 5 minutes before serving.