Preheat oven to 350°. Grease a 1-1/2-quart casserole.

Cook macaroni in large saucepan according to package directions. Pasta should be tender, but not too soft. Drain well and allow to cool slightly. Return to saucepan.

Stir in butter, mustard, egg, and salt. Add 2-1/2 cups cheese and flour, tossing well to evenly distribute ingredients. Turn mixture into prepared casserole dish. Pour milk over macaroni and cheese. Use the back of a spoon to press down ingredients and level the top. Bake for about 35 minutes. Sprinkle remaining cheese on top and bake about 10 minutes longer or until the custard is set and the top is golden and crusty.