Boil pasta until just done. Drain. Meanwhile make the cheese sauce by melting butter in a large saucepan.

Stir in flour and cook 1 minute, incorporating flour to form a roux. Slowly add milk, stirring to incorporate. When all milk has been added, stir until sauce is smooth and has thickened.

Season with salt and pepper. Add 4 oz. of the cheddar and all of the Parmesan and stir until melted.

Add pasta to sauce and stir to combine with sauce. Pour pasta and sauce into a seasoned iron skillet. Top with remaining cheddar. Place on grill with lid down and cook over medium coals, about 350 degrees for 45-60 minutes.