Sauce Mornay — Gruyère and Egg Yolk Béchamel

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Mornay is béchamel enriched with grated Gruyère (or a blend of Gruyère and Parmesan) and finished with egg yolks — the gratin sauce, the croque monsieur sauce, the reason cauliflower cheese exists. It begins where béchamel ends: a smooth, medium-thick white sauce made from a blonde roux and scalded milk. Off heat, egg yolks are tempered in — a spoonful of hot sauce whisked into the yolks first, then the yolk mixture returned to the pot — followed by finely grated cheese stirred in handfuls until each addition melts completely before the next goes in. The finished sauce should coat a spoon thickly, pull in slow sheets when lifted, and taste of cheese without tasting of raw flour or scorched milk. The cheese selection is where the dish lives or dies. Gruyère provides melt and nuttiness; Parmesan adds salinity and depth. A 3:1 ratio of Gruyère to Parmesan is classical. Pre-grated cheese contains cellulose anti-caking agents that prevent smooth melting — grate it yourself. The cheese goes in off direct heat or over the barest flame: above 70°C the casein proteins in cheese tighten and squeeze out fat, producing a grainy, oily sauce. This is the most common failure mode. Egg yolks serve two purposes: they enrich the body and they enable gratinéeing. A Mornay without yolks will brown under a salamander, but it will not achieve the blistered, mahogany crust that defines a proper gratin. The yolks provide proteins that undergo Maillard reactions at high surface temperatures, creating the characteristic flavour of browned cheese. For a gratin: pour Mornay over blanched vegetables, fish, or pasta in a gratin dish. The sauce layer should be 8-10mm thick. Flash under a preheated salamander or broiler at maximum heat — 2-3 minutes — until the surface blisters and colours unevenly. The sauce beneath should still be fluid; if the gratin sits too long under heat, the sauce breaks and the dish becomes greasy. Mornay is the direct ancestor of every baked pasta dish in the French canon and the unacknowledged foundation of British and American 'mac and cheese', which is simply Mornay over elbow macaroni. In Italian cooking, besciamella with Parmigiano serves an identical structural role in lasagne.

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