Mornay Sauce

Melt the butter over medium-low heat until it foams but does not brown. Whisk in the flour all at once and cook, stirring constantly, for 2-3 minutes to cook out the raw-flour taste while keeping the roux pale. Keep it white — a Mornay should taste of cheese, not toasted flour.

Stream in the warm milk, whisking hard and constantly — the first third slowly to build a smooth paste, then the rest in a steady pour. Constant whisking is what keeps it lump-free.

Simmer gently 8-10 minutes, whisking often, until the sauce thickens to nappe consistency — thick enough to coat the back of a spoon. Build it slightly thicker than a plain béchamel; the cheese will loosen it later. Season lightly with salt, nutmeg, and white pepper, remembering the cheeses bring their own salt.

Off the heat — and this matters, because boiling cheese turns greasy and stringy — whisk in the Gruyère and Parmigiano a handful at a time until each melts. Temper the yolk: whisk a ladle of the warm sauce into the yolk, then whisk that back into the pot. The residual heat cooks and thickens the yolk without scrambling it. Adjust salt last.