Many modern cheese sauces owe their composition to the original French Mornay. Traditional French cookery identifies Mornay as a “small sauce” based on Bechamel, which is made with flour, butter and milk or cream. A true Mornay sauce uses 4 ounces of grated Gruyere or strong Swiss cheese combined with 2 ounces of grated Parmesan for every quart of Bechamel. One professional technique finishes the sauce with fresh butter and thins it with hot milk or broth. More modern versions sometimes rely on different cheeses, such as cheddar.

Holding a sauce involves keeping it warm and ready to use without changing the consistency or quality of the finished product. With some sauces, such as tomato or espagnole, this process can be relatively easy. The butter, cheese and milk in Mornay all make it difficult to hold the sauce for long periods of time, however. “Professional Cookery” recommends holding sauces at around 165 degrees Fahrenheit in a double boiler to prevent scorching. If you attempt to hold Mornay sauce overnight, you must maintain a consistent temperature the entire time to keep it from separating, curdling or burning. Stir the sauce periodically and cover it to reduce the risk of over-thickening.

Mornay sauce's delicate nature means that you'll have better luck cooling it for overnight storage. The best way to do this involves placing the entire container into a cold water bath and bringing the temperature of the sauce down in a controlled manner. You can pour melted butter over the surface of the partially cooled sauce or cover it with a piece of plastic wrap. These techniques keep the sauce from forming an unappealing skin. Store the sauce in the refrigerator and reheat it in a double boiler just before you need to serve it.

While time pressures might require you to make Mornay sauce ahead, this food does best when made fresh. If possible, make the entire sauce right before you need to serve it. When this isn't an option, you can make the basic Bechamel the night before, then cool and store it just as you would a Mornay, then reheat it shortly before you need the sauce. Add the cheese, butter and stock to the hot Bechamel as though you were making a completely fresh sauce. This technique saves time while still preserving the best properties of the ingredients.