Boil the Pasta: Bring about 4 quarts of water to a boil over high heat in the pasta pot. Add the pasta and a tablespoon of salt. Cook until the pasta is al dente, about 8 minutes. Drain and set aside.

Warm the Milk: When the pasta has finished cooking, prepare the cheese sauce. Begin warming 1 cup of the milk in the saucepan over medium heat. Whisk together the remaining 1/2 cup of milk and the flour until there are no lumps. When you just start to see tendrils of steam rising from the warming milk, whisk in the milk-and-flour mixture. Continue whisking gently until the milk thickens slightly to the consistency of heavy cream, 3-4 minutes.

Make the Cheese Sauce: Turn the heat to low and begin mixing handfuls of cheese into the milk. Stir in the salt and mustard. Stir until all the cheese has melted and the sauce is creamy. Taste and adjust the seasonings as desired. Remove the sauce from heat.

Combine the Pasta and Cheese Sauce: In a large serving bowl, combine the pasta and 1/2 of the cheese sauce. Stir to coat the pasta evenly. Add the second half of the sauce and any extra add-ins.

Serving and Storing Leftovers: Serve the mac and cheese immediately while still warm. Leftovers will keep for up to a week and can be reheated in the microwave. If the sauce is a little dry after re-heating, mix in a splash of milk to make it creamy again.