For special occasions , chicken or goose liver , along with veal . Today , both butter and olive oil are used for cooking the Soffritto of small amounts of celery , carrot and onion .
However , different recipes , even in the Bolognese tradition , make use of chopped pork or pork sausage , white chicken , rabbit , or goose liver may be added along with the beef or veal for special occasions , and today many use both butter and olive oil for cooking the soffritto of small amounts of celery , carrot and onion .