Sauce Mornay

The science

The béchamel's starch network is what makes Mornay reliable where a bare cheese sauce would break. Cheese proteins (casein) want to clump and squeeze out fat and water when overheated; the gelatinized starch coats and separates them, holding the emulsion of fat and water stable and keeping the sauce glossy rather than stringy or oily. This is why the cheese goes in off the heat or over the gentlest warmth and is stirred just until melted — pushing it to a hard simmer recoagulates the casein and "breaks" the sauce into grease and curds.

Flavor profile

Gruyère and Parmesan (sometimes Emmental or comté), melted into warm medium béchamel off direct heat, frequently with an egg-yolk enrichment.

Culinary uses

Croque-monsieur, gratins, œufs Mornay, cauliflower and sole gratins, and as the upscale ancestor of macaroni-and-cheese.

Reference notes

What goes wrong

Adding too much cheese (the starch can only stabilize so much fat), or overheating after the cheese is in.