Combine 1 cup (250 milliliters) of mushroom cooking liquid with 2 cups (500 milliliters) of reduced-cream white sauce. Reduce the mixture until it has the desired final consistency; add 1 cup (250 milliliters) of Sauce Soubise I. Thin the sauce with stock or reduce it slightly if necessary to adjust its consistency, and finish with 1½ ounces (45 grams) of butter.