To make cheese sauce, melt butter in 1-quart saucepan over medium-high heat. Stir in flour; cook about 1 minute. Beat in milk and mustard with whisk. Heat to bubbling, and let thicken, about 2 minutes. Turn off heat, and stir in provolone cheese, Cheddar cheese and a pinch of salt and pepper. Stir until cheese is melted.