Preheat the oven to 350 F (175 degrees C).
For a crispy skin, dry the bird with a paper towel before seasoning.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.
HOW TO STORE LEFTOVER CHICKEN
Allow your leftover roast chicken to cool completely, then store it in an airtight container in the refrigerator for up to four days. Reheat thoroughly in the oven or in the microwave.