01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - In a large skillet over medium heat, cook celery, onion, and carrots in a small amount of oil or butter for 5-7 minutes until softened. Remove from heat.
03 - Whisk together cream of chicken soup, sour cream, milk, garlic powder, thyme, pepper, and salt in a large bowl until completely smooth and combined.
04 - Add diced chicken and sautéed vegetables to the sauce mixture. Fold gently until evenly distributed throughout the creamy base.
05 - Transfer the chicken and vegetable mixture to the prepared baking dish, spreading it into an even layer across the bottom.
06 - Combine stuffing mix, melted butter, and chicken broth in a separate bowl. Mix well until all stuffing crumbs are evenly moistened but not soggy.
07 - Sprinkle the prepared stuffing mixture evenly over the chicken layer, covering the entire surface.
08 - Bake uncovered for 35-40 minutes until the stuffing topping is golden brown and the filling bubbles around the edges.
09 - Let the casserole rest for 5-10 minutes before serving to allow the layers to set for easier scooping.