Put the stock and beef mince in a heavy saucepan. Add the tomatoes, carrots, onion, celery, turnip, bouquet garni, egg whites, egg shells and plenty of seasoning.
Bring almost to the boiling point, whisking all the time with a flat whisk.
Cover and simmer for 1 hour, taking care not to allow the layer of froth on top of the soup to break.
Carefully pour the soup through a scalded fine cloth such as muslin, keeping the froth back. Repeat if necessary until the liquid is clear. Add the sherry and reheat. Serve hot.