Melt the butter in heavy saucepan.
If using all purpose flour, stir the flour into the butter first, and then add milk. If using Wondra, stir the Wondra flour into the milk, and then add milk mixture to the butter.
Stir in the mustard, salt, and pepper.
Stirring constantly over medium heat, cook until the sauce begins to thicken and almost comes to a boil.
Add the grated cheeses and slowly stir until the cheese melts and the sauce becomes velvety and smooth.
Remove from heat and serve over steamed or roasted vegetables, or use as a dip for chips, shrimp, etc.