Brown the mushrooms undisturbed and salt them last — that single step is the difference between a deep, savory sauce and a pale one. Keep the sauce at a gentle simmer once the cream is in; a hard boil will split it. To make Chicken Friand instead, dice the cooked chicken into half the sauce, chill until firm, wrap in puff pastry squares, and bake at 400°F for 20 to 25 minutes.