01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter or cooking spray.
02 - In a large skillet over medium heat, sauté celery, onion, and carrots in a drizzle of oil or butter for 5–7 minutes until just softened.
03 - In a large bowl, whisk together cream of chicken soup, sour cream, milk, garlic powder, thyme, pepper, and salt until smooth and well combined.
04 - Fold the cooked chicken and sautéed vegetables into the sauce mixture until evenly distributed.
05 - Spread the chicken and vegetable mixture evenly into the prepared baking dish.
06 - In a separate bowl, combine stuffing mix, melted butter, and chicken broth. Mix until stuffing is evenly moistened but not soggy.
07 - Sprinkle the stuffing mixture evenly over the chicken layer, covering completely.
08 - Bake uncovered for 35–40 minutes, or until the top is golden brown and the filling is bubbling around the edges.
09 - Let the casserole rest for 5–10 minutes before serving to allow layers to set.