Preheat your oven to 350°F (175°C).

Lightly spray a 9×13-inch baking dish with nonstick cooking spray.

Season the boneless, skinless chicken breasts with salt and pepper. Place them at the bottom of your prepared baking dish.

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

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

Spread the stuffing mixture evenly over the seasoned chicken in the baking dish.

Cover the dish with foil and bake for 30 minutes. In the last 15 minutes, remove the foil to allow the top to brown.

Once out of the oven, let the casserole sit for about 10 minutes before serving.