1. Microwave milk in a 2-cup microwave-safe glass measuring cup at HIGH 1 minute.
2. Melt butter in a small heavy saucepan over medium heat; gradually whisk in flour. Cook 1 minute, whisking constantly. Gradually whisk in warm milk; cook over medium heat, whisking constantly, 3 to 5 minutes or until thickened and bubbly. Remove from heat; whisk in cheese, salt, and pepper.
Cheddar Cheese Sauce: Substitute extra-sharp white Cheddar cheese for Gruyère. Proceed with recipe as directed.