Bring 2 quarts of water to boil in large soup kettle.
Add salt and pasta. Using package directions as a guide, cook until al dente.
Drain. To prevent sticking, drain and immediately pour onto a large lipped baking sheet and let cool while preparing sauce.
Meanwhile, microwave the milk and chicken broth in a 4-cup Pyrex measuring cup or bowl until hot and steamy (not boiling). Melt butter in the empty pasta pot; whisk in flour, then hot milk mixture. Continue to whisk until thick and bubbly, 3 to 4 minutes.
Whisk in mustard, Parmesan and pepper. Turn off heat, stir in cheddar until melted.
Add drained pasta (and optional flavoring ingredients, see notes) to sauce, and stir until everything is well combined over low heat. Stir to heat through, and thin with a little water if the sauce is too thick.
