Whisk milk and flour together in a medium saucepan. Bring to a boil over medium heat, stirring frequently. Remove from heat; add mustard. For a smooth dressing, add all of the cheese and stir until melted. Or, for a chunkier dressing, reserve 1/4 cup of the cheese to add to cooled dressing. Let cool to room temperature. Whisk in mayonnaise season to taste. For optimum flavor, serve at room temperature, but store in refrigerator. If desired, add crumbled crisp-cooked bacon, or finely chopped chives to dressing.
*When shredding cheese, to prevent sticking, moisten shredder with water or lightly spray with cooking spray.