Preheat your oven to 350°F (175°C) and lightly spray a 9 x 13-inch baking pan with cooking spray.
In a medium saucepan, bring the water or chicken broth to a boil. Add in the stuffing mix, remove from heat, cover, and let it sit. Fluff with a fork when done.
In a skillet, heat up the butter and sauté the diced onion and celery until softened.
In a bowl, mix together the cream of chicken soup, cream of mushroom soup, and sour cream.
Place the shredded chicken at the bottom of the prepared baking dish. Layer the cooked onions and celery over the chicken, then sprinkle with salt and pepper.
Pour the soup and sour cream mixture over the onion and celery layer.
Top the entire casserole with the prepared stuffing mix.
Bake in the preheated oven for 1 hour, or until the casserole is bubbly and browned on top.
Let the casserole cool for a few minutes before serving. Enjoy!
