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1
Bring the velouté to a steady simmer in a heavy saucepan, stirring occasionally, until it coats a spoon thickly. Remove from direct heat. The base must be unctuous and fully reduced before the liaison is prepared.
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2
In a bowl, whisk the egg yolks with the heavy cream — and the cornstarch, if using — until the mixture is completely uniform, pale, and shows no streaks of yolk.
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3
This is where the dish lives or dies. Ladle approximately 100 ml of the hot velouté into the yolk-cream mixture in a thin, steady stream, whisking without pause. Add a second ladle in the same manner. The liaison must be brought gradually to the temperature of the sauce — never the reverse. The tempered mixture should feel warm to the back of the wrist but not hot.
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4
Return the tempered liaison to the pot of velouté in a thin stream, whisking constantly. Set over low heat. Stir without stopping until the sauce thickens enough to coat a spoon heavily — you will feel the viscosity increase through the whisk. At no point must the sauce approach a simmer; egg yolks coagulate above 70°C and will produce visible curds if the temperature is exceeded.
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5
Remove from heat. Add the lemon juice in drops, tasting between each addition, until a clean high note lifts the sauce without sharpness. Add a few gratings of fresh nutmeg. The acid must not be rushed — too much will curdle the cream component.
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6
Pass immediately through a fine chinois into a warm serving vessel. The finished sauce should be pale gold, glossy, and thick enough to nap a piece of meat without running — the surface will hold a spoon's impression for a moment before settling.