01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with cooking spray or butter.
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 softened.
03 - In a large bowl, whisk together cream of chicken soup, sour cream, milk, garlic powder, thyme, pepper, and salt until thoroughly combined and smooth.
04 - Add the cooked chicken and sautéed vegetables to the sauce mixture. Fold gently until evenly distributed.
05 - Transfer the chicken mixture to the prepared baking dish, spreading it into an even layer.
06 - In a separate bowl, combine stuffing mix, melted butter, and chicken broth. Mix until the stuffing is evenly moistened.
07 - Sprinkle the stuffing mixture evenly over the chicken layer, covering the entire surface.
08 - Bake uncovered for 35–40 minutes until the top is golden brown and the filling is bubbling around the edges.
09 - Remove from oven and let the casserole rest for 5–10 minutes before serving to allow the layers to set.