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 with a drizzle of oil or butter. Add celery, onion, and carrots. Cook for 5-7 minutes until softened but still slightly crisp. Remove from heat.
03 - In a large bowl, whisk together cream of chicken soup, sour cream, milk, garlic powder, dried thyme, black pepper, and salt until completely smooth and well combined.
04 - Add the cooked chicken and sautéed vegetables to the sauce mixture. Fold gently until evenly distributed throughout the creamy base.
05 - Spread the chicken and vegetable mixture evenly into the prepared baking dish, pressing lightly to create an even layer.
06 - In a separate bowl, combine the seasoned stuffing mix with melted butter and chicken broth. Mix thoroughly until all stuffing pieces are evenly moistened but not soggy.
07 - Sprinkle the prepared stuffing mixture evenly over the chicken mixture, covering the entire surface.
08 - Bake uncovered for 35-40 minutes until the stuffing is golden brown and the filling is bubbling around the edges. If the topping browns too quickly, cover loosely with foil.
09 - Remove from oven and let the casserole rest for 5-10 minutes. This allows the filling to set slightly for easier serving.