Cover the duck carcass, gravy, drippings (after removing as much fat as possible), and stuffing with water. Add onion, carrot, turnip, celery, the bay leaf, salt, and peppercorns. Simmer all together for an hour or more until the meat remaining falls easily from the bones. Remove the carcass from the pot, pick off the meat, and return the latter to the soup. Cool the soup and skim off the fat. Reheat the soup and thicken with mashed potato or potato flour (amount used will depend on thickness desired). Correct the seasoning, adding Worcestershire or soy sauce if needed, sliced mushrooms, and chopped parsley.