Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 2-quart baking dish.
Boil macaroni in a large pot of salted water until barely done, about 5 minutes, or 7 minutes for gluten free pasta. Drain and set aside.
Whisk together milk and egg in a large bowl. Stir in cheese, mustard, and butter.
Place par-boiled macaroni in the prepared baking dish. Pour milk mixture over macaroni, season with salt and pepper, and stir until combined. Press mixture evenly into the baking dish.
Bake, uncovered, in the preheated oven until the top is browned, 30 to 40 minutes.
Serve hot and enjoy!