Preheat Oven: Preheat to 375°F and grease a 9×13-inch baking dish with cooking spray or butter.
Sauté Aromatics: Melt butter in a skillet over medium heat. Add onion and celery; sauté for 3-4 minutes until fragrant and tender.
Combine Base: In a large mixing bowl, combine chicken, sautéed vegetables, frozen vegetables, cream of chicken soup, chicken broth, garlic powder, thyme, salt, and pepper. Mix well.
Spread Mixture: Spread evenly in the baking dish, pressing down slightly to create a solid base.
Prepare Stuffing: Follow package instructions to prepare stuffing mix, then let it rest for 5 minutes.
Top Casserole: Spoon stuffing evenly over chicken mixture, covering the surface completely.
Bake: Bake uncovered for 30–35 minutes until the top is golden brown and edges are bubbling. Internal temperature should reach 165°F.
Rest: Let sit for 5 minutes before serving to allow flavors to settle and texture to firm.