Bring 1 gallon of water that has been salted to a boil. Then add pasta. Cook until al dente.
Melt the butter in large saucepan. Add the flour, mustard, salt, and pepper. Stir to combine. Cook until all the flour has mixed in and the mixture takes on a little brown color.
Add milk. Bring to a boil. Stir constantly for 1 minute.
Slowly add in the cheese. Mix until smooth.
Pour over pasta. Serve immediately for best results. Or you can bake it in a 350 degree oven for 25-30 minutes to get a nice brown crust on top.