While the meatloaf is baking, melt the butter in a saucepan over medium heat. Stir in the flour and cook for 1-2 minutes to make a roux.
Gradually whisk in the milk and bring to a simmer. Cook until thickened, about 3-4 minutes.
Stir in the shredded cheddar cheese, mustard powder (if using), salt, and pepper. Mix until the cheese is fully melted and the sauce is smooth.
Add the cooked macaroni to the cheese sauce and stir until well coated.
Assemble the Casserole:
