Preheat your oven to 350°F (175°C). Spray a 9×13-inch baking dish with nonstick cooking spray.

Season chicken breasts with salt and pepper; lay them at the bottom of the baking dish.

In a skillet over medium heat, melt the butter. Sauté diced onion and celery until translucent, about 5 minutes.

In a large bowl, combine stuffing mix, cream of chicken soup, chicken broth, sautéed vegetables, poultry seasoning, and black pepper. Stir until well combined.

Spread stuffing mixture evenly over the chicken in the baking dish, ensuring every piece is covered.

Cover with foil and bake for 30 minutes. Remove the foil and bake for an additional 15 minutes for a golden crust.

Let the casserole rest for about 10 minutes before serving.