Place chicken in a roasting pan and season generously inside and out with salt and pepper.
Sprinkle inside and out with onion powder.
Place 3 tablespoons butter in the chicken cavity along with the celery.
Arrange dollops of the remaining butter around the chicken's exterior and under the skin.
Bake uncovered 1 hour 15 minutes in the preheated oven, to a minimum internal temperature of 180 degrees F.
Remove from heat and baste with melted butter and drippings.
Cover with aluminum foil, and allow to rest about 30 minutes before serving.