While the water is coming up to a boil or macaroni is cooking, you can start to heat your cast iron skillet over medium heat and melt the butter.
½ cup Salted Butter
Whisk in the flour until smooth (about 3-4 minutes) to make a roux.
⅓ cup All Purpose Flour
When the flour mixture has cooked a bit and taken on slight color, whisk in the salt, pepper and mustard powder.
1 tablespoon Kosher Salt, ¼ teaspoon Freshly Ground Black Pepper, ½ tablespoon Mustard Powder
Very slowly add the milk, whisking constantly. Just add small amounts of milk at a time and add more once the previous amount is completely incorporated. Make sure to whisk the sides and bottom of the pan to prevent the sauce from sticking.
4 cups Whole Milk
Bring this white sauce, known as a bechamel, to a simmer, then reduce the heat and continue to whisk until it has thickened, about 8-10 minutes total.
Remove the skillet from the heat, then add the shredded cheese, a little at time in small handfuls, whisking until smooth. Continue whisking until all of the cheese has been incorporated and you have a smooth cheese sauce (this is a variation of a Mornay sauce).
4 Cups Sharp Cheddar Cheese, 2 Cups Gruyere Cheese