Melt butter in a 2-quart baking dish. Pour uncooked macaroni into melted butter and stir until coated. Add mustard and stir to blend.
Add salt, pepper, cheese slices and cold milk to macaroni. Do not stir. Bake uncovered at 325 degrees for 1 1/2 hours. Do not stir while baking. The macaroni and cheese will be golden brown and creamy.