Boil the stock, and add to it the sauerkraut, sliced potatoes, bacon, mushrooms, spices and salt. Simmer for ¾ hr. or until the sauerkraut is quite soft. Rub through a sieve. Reheat, adding the sugar and seasoning and the sausages cut into ¼-in. thick rounds. Simmer the soup 15–20 min. until the sausage is cooked. Add the cream if liked. Sprinkle cheese on top or serve separately.