Peel, clean, wash and chop the carrots, parsnip and rutabaga. Peel and chop the onions. Heat the oil in a wide saucepan. Fry vegetables, onions and bay leaf for 2-3 minutes.
Add the stock and simmer gently for 15-20 minutes. Carefully remove fat and bones from the knuckles. Cut meat into pieces and add to the soup about 5 minutes before the end of the cooking time. Wash parsley and dab dry.
Set aside 4 stems for garnishing. Remove the rest of the leaves and chop finely. Arrange the soup in deep plates and sprinkle with parsley. Garnish with 1 parsley stalk each. Serve with mustard.