Mornay sauce is a type of bechamel or white sauce, one of the five French mother sauces. Classic mornay sauce is a bechamel sauce with cheese. Gruyere or swiss cheese is the most common. Sometimes sherry is also added to brighten the flavor. In addition to topping these chicken crepes, the mornay sauce can also be used as a topping for vegetables, pasta, seafood, and chicken. My recipe for Crab Mornay also features this wonderful cheesy sauce.