Cheese sauce is one of the tastiest sauces to put on pasta. It is thick, delicious, and very rich. Try rich cheeses like Gruyere or use a milder cheese for a milder sauce. The Fontina cheese in this recipe is very important; it is adds an essential flavor. Use this recipe as the base for the four-cheese sauce of your dreams. The base of this sauce is called a roux; most roux recipes are very similar and quite simple. The essential step to making a thick, smooth sauce is to make sure the butter and flour are smooth before you add the milk.