Cook macaroni according to package directions; drain well. Set aside.
Place milk, flour, and onion salt in a quart jar; cover tightly, and shake vigorously 1 minute.
Stir together flour mixture, 3 1/2 cups cheese, and macaroni.
Pour macaroni mixture into a lightly greased 13- x 9-inch baking dish or 2 (11-inch) oval baking dishes. Sprinkle evenly with breadcrumbs and remaining 1 cup cheese; drizzle evenly with melted butter.
Bake at 350° for 45 minutes or until golden brown.
*20 ounces loaf pasteurized prepared cheese product, shredded or cut into small cubes, may be substituted. Omit breadcrumbs if using prepared cheese product.
Note: For testing purposes only, we used Kraft Cracker Barrel Sharp Cheddar Cheese.