Meanwhile, pour oil into a pan over medium heat. Add the onions and saute for 3 to 5 minutes. Add the celery, carrots, fennel, beets and potatoes, cover the pan and cook until tender. Let the steam out of the pan once in a while.
Using the pan the duck breasts were cooked in, discard any fat and add the cider and Calvados. Cook over medium heat to reduce the mixture by half and add the veal stock. Just before you are ready to serve, add the butter to the mixture--this will create a nice sauce.