A Mornay Sauce is a Béchamel sauce with shredded or grated cheese added. Usually, it is half Gruyère and half Parmesan, though variations use different combinations of Gruyère, Emmental, or white cheddar.

It is often served with seafood or vegetables.

Which duc de Mornay, if any, is honored in sauce Mornay is not usually debated:

Philippe, duc de Mornay (1549-1623), Governor of Saumur, and seigneur du Plessis-Marly, writer and diplomat, is generally the favored candidate, but a cheese sauce at his table must have been based on what is called today a velouté sauce, for Béchamel had not been invented. Perhaps sauce Mornay is not older than the great Parisian restaurant of the 19th century, Le Grand Véfour in the arcades of the Palais-Royale, where sauce Mornay was introduced. In the tout-Paris of Charles X, the Mornay name was represented by two extremely stylish men, the marquis de Mornay and his brother, styled comte Charles; they figure in Lady Blessington's memoir of a stay in Paris in 1828-29, The Idler in France, (1841). They might also be considered, when an eponym is sought for sauce Mornay.