Simmer celery in boiling water, covered, 15 to 20 min or until tender. Drain, reserving the liquid. Crumble and fry the sausage. Drain off fat. Clean pan. Melt butter in pan and slowly cook onion until tender but not brown. Saute mushrooms with some olive oil until most moisture is out. Add enough of the celery water for the stuffing to be moist. Mix all ingredients in a large bowl and stuff neck and body cavity of a 15-20 lb bird.