Heat milk and stir in roux to thicken. Dissolve salt, mustard and nutmeg in a little water and add to thickened mixture. Reduce heat. Gradually stir in with a wooden spoon all the cheese. Continue to cook, stirring constantly until cheese has melted and sauce is smooth. Mix with elbow macaroni, top with bread crumbs and bake in a preheated 400 degree oven until browned and bubbly.