Mornay sauce is a Béchamel sauce enriched with egg yolks and Gruyère cheese (and/or sometimes Parmesan). It is mainly used with fish, egg and vegetable dishes; frequently browned in the oven or under the grill.
There are generally two stories told about the origin of the name 'Mornay'. One is that it was named for a 17th century French diplomat, Philippe de Mornay, seigneur du Plessis-marly (1549-1623). The other is that it was created by Chef Joseph Voiron of Paris, who named the sauce for his eldest son, who was known as 'Mornay'.