Cook macaroni, drain and rinse, and set aside. Cube cheese.

Preheat oven to 350.

Heat milk in a heavy pan while stirring in gluten free roux cubes until they melt and the sauce begins to thicken. Add the mustard and horseradish, and stir.

Add the cheese, part at a time, stirring (to prevent scorching) while it melts smoothly.

Once you have a smooth cheese sauce, remove from heat, stir in the macaroni. Then turn it into a one quart baking dish. Sprinkle crumbs (if using) over the top.

Bake at 350 for 15 minutes.

Remove from oven and let rest for five minutes before serving.