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.