Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine shredded chicken, sour cream, and cream of chicken soup.
Add the diced celery and onions to the mixture.
Sprinkle in garlic powder, salt, and pepper; mix well.
In another bowl, prepare the stuffing mix according to package instructions using chicken broth.
Fold the stuffing mix into the chicken mixture until well combined.
Transfer the mixture to a greased 9x13-inch baking dish.
Drizzle melted butter over the top of the casserole.
Bake in the preheated oven for about 30 minutes, or until golden brown.
Remove from the oven and let it cool for a few minutes before serving.