Combine 1½ cups (375 milliliters) of mushroom cooking liquid with 1½ cups (375 milliliters) of concentrated white stock. Reduce this mixture slightly if necessary to concentrate the flavor of the mushrooms. Combine the mushroom-stock mixture with 10 to 12 beaten egg yolks, as described for Egg Yolk–Thickened White Sauce, and gently cook over low heat while stirring until the sauce turns silky and thickens. Remove from the heat and continue stirring for a couple of minutes to let the sauce cool and to prevent it from curdling. Finish the sauce with 3 ounces (90 grams) of butter, 2 teaspoons (10 milliliters) of lemon juice, and 2 tablespoons (30 milliliters) of finely chopped parsley.