1. Preheat oven to 350 F. Cook the macaroni in boiling water until al dente. Drain the macaroni and return to saucepan. Stir in the butter and the egg.
2. In a large bowl, combine the mustard, salt, garlic salt, and nutmeg with the tablespoon of boiling water. Stir in the milk, 2 1/2 cups of the cheese, onion, and macaroni.
3. Pour the macaroni into a lightly greased baking dish. Top with remaining cheese and sprinkle with paprika. Bake 1 hour or until the top is all light brown and crusty.