Preheat oven to 350°. Lightly grease or PAM a 1 1/2-quart casserole dish. Cook the macaroni or pasta in boiling water following the package directions until tender but slightly firm to the bite.
Drain the pasta or macaroni; return pasta to saucepan. Stir in the butter and egg. In a large bowl, combine mustard, salt and nutmeg with the 1 tablespoon of water. Stir in the milk, 2-1/2 cups of the cheese, the onion and pasta.
Pour the macaroni mixture into the prepared casserole dish. Top with the remaining cheese sprinkle with the paprika. Bake 350° for 1 hour or until topping has browned.
