Preheat the oven to 400 if you are baking your mac and cheese.
Bring a large pot (about 3 quarts) of water to a boil and add 1 tablespoon of salt. Add the pasta and set a timer for two minutes less than the box suggests. If it gives a range, use the shortest time. If it gets done before the sauce, just drain it and set it back in the pot until you need it.
Once the pasta is in, start the sauce.In a 12-inch skillet, melt the butter over medium high heat.
Add the flour and stir to cook out the raw flour flavor--about 1 minute. Add the milk (warmed if possible) and stir to combine. Add 1/2 teaspoon salt and 1/4 teaspoon pepper and reduce heat to medium low.
Cook until the sauce simmers, stirring constantly, and the sauce is thick enough to coat the back of a spoon--about 5-6 minutes.
Turn off the heat and add the mustard, ham and sage. Add the cheeses. Stir until the cheese melts. Add the pasta.
Transfer to a greased 8x8 or slightly larger baking dish, or bake it in the skillet if your pan is safe for the oven. Bake until hot throughout, 20-25 minutes.
If you want to broil the top, preheat the broiler and broil--about 5 inches from the element or flame--5 minutes or so. Keep an eye on it so you don't burn it!
Notes
For my gluten free friends, I find great results with Barilla gluten free pasta and King Arthur's All Purpose Gluten Free Baking Mix.