01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter or cooking spray.
02 - Heat a large skillet over medium heat. Add a drizzle of oil or butter, then sauté celery, onion, and carrots 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 completely smooth.
04 - Add cooked chicken and sautéed vegetables to the sauce mixture. Fold gently until evenly coated.
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 all stuffing is evenly moistened.
07 - Sprinkle the stuffing mixture evenly over the chicken layer, covering completely.
08 - Bake uncovered for 35–40 minutes 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 the filling to set.