Preheat oven to 350°. Lightly grease a 1-1/2 quart casserole dish. Cook the macaroni or pasta in boiling water following the package directions until al dente, or still a bit firm to the bite. Drain the pasta or macaroni, then return it to the saucepan. Stir in the butter and egg. In a large bowl, combine mustard, salt and nutmeg with the 1 tablespoon of boiling water. Stir in the milk, 2-1/2 cups of the cheese, the onion and macaroni or pasta. Pour the macaroni mixture into the prepared casserole dish. Top with the remaining cheese. Sprinkle with the paprika. Bake the casserole in the 350 degree oven for 1 hour or until a well- browned top crust has formed.Serves 4.