Combine 2 cups (500 milliliters) white veal, chicken, or other white stock; 1 cup (250 milliliters) mushroom cooking liquid; and 1 tablespoon (15 milliliters) lemon juice. In a medium saucepan, reduce this mixture to 1 cup (250 milliliters). Combine with 1 quart (1 liter) sauce velouté and reduce back to 1 quart (1 liter) or, better, until the sauce takes on the desired consistency. Whisk together 5 egg yolks and pour half the velouté mixture into the yolks while whisking. Return the egg yolk mixture to the saucepan, place over low to medium heat, and stir constantly with a wooden spoon until the liquid takes on a silky consistency in much the same way as a crème anglaise. By no means let the sauce boil. Whisk 4 ounces (120 grams) cold butter into the sauce.
Notice that, contrary to what’s often called for in traditional recipes, the components to this sauce are reduced before adding the egg yolks. Sauces containing egg yolks can only be boiled if they contain a large percentage of flour. The flour stabilizes the sauce so the yolks don’t break. Pastry cream is an example of this principle. When translating traditional sauce recipes containing flour into flourless versions, the egg yolks should be added only at the end; the sauce should never be allowed to boil.