Preheat the oven to 350 degrees F (175 degrees C).

Place chicken in a roasting pan; season generously inside and out with onion powder, salt, and pepper. Place 3 tablespoons of butter in chicken cavity; arrange dollops of remaining butter on the outside of chicken. Cut celery into 3 or 4 pieces; place in the chicken cavity.

Bake chicken uncovered in the preheated oven until no longer pink at the bone and the juices run clear, about 1 hour and 15 minutes. An instant-read thermometer inserted into the thickest part of the thigh, near the bone, should read 165 degrees F (74 degrees C).

Remove from the oven and baste with drippings. Cover with aluminum foil and allow to rest for about 30 minutes before serving.