| Preheat oven to 350 degrees F. Place chicken in a roasting pan, and season generously inside and out with onion powder, salt and pepper. Place 3 tablespoons butter in the chicken cavity. Stuff the remaining butter under the chicken skin. Cut the celery into 3 inch pices and place in the chicken cavity. Place onion wedges and mushrooms around the outside of the chicken. Bake uncovered 1 hour 15 minutes, to a minimum internal temperature of 180 degrees F. Remove from oven and baste with pan drippings. Cover with aluminum foil and allow to rest for 30 minutes before serving. | 
