Preheat oven to 350°F (175°C)
Wash the chicken, pat dry with a paper towel.
Place chicken in a roasting pan, and season generously with salt, pepper, and onion powder inside out. Sprinkle on every part, not forgetting the cavity.
Place 3 tablespoons of butter chunks inside a chicken cavity, then stuff the cavity with celery cut into 3-4 pieces.
Melt the rest of butter and sprinkle on the chicken using a silicon brush. Sprinkle the melted butter trying not to touch the skin. As soon as the butter touches the cold chicken skin, it hardens. Then you can use the brush to spread the remaining melted butter on the chicken. Sprinkle some more salt and pepper if needed.
Bake uncovered for 1 hour and 30 minutes in the preheated oven. Remove from heat and baste with drippings using a spoon or a ladle. Cover the pan lid. If there is no lid, then cover with (unbleacheparchment paper and cover the parchment paper with aluminum foil. It's better if aluminum foil does not touch the chicken directly because we don't want aluminum traces on our meat. Put the covered chicken back to the oven and bake for 20 minutes more. Allow to rest for an additional 20 minutes still covered outside the oven.