Preheat the Oven: Set your oven to 375°F (190°C/Gas Mark 5). Grease a 9x13 inch baking dish with oil or non-stick cooking spray.
Prepare the Stuffing: Heat 1½ cups of chicken broth in the microwave for 2 minutes or in a small saucepan until it’s nearly boiling. In a large mixing bowl, combine the stuffing mix with the hot broth, 2 tablespoons of butter, and ½ cup of milk. If using, add ½ cup of diced white onion, ½ cup of celery, and/or ½ cup of dried cranberries. Mix everything well and set aside.
Prepare the Chicken: Trim any excess fat from the chicken breasts. Using a sharp knife, carefully cut each breast ¾ of the way through (like butterflying) to create a pocket for the stuffing. Be sure not to cut all the way through.
Stuff the Chicken: Spoon ¼ of the stuffing mixture into each chicken breast pocket. Place the stuffed breasts in the prepared baking dish. Season each breast with the salt, pepper, and onion powder mixture.
Bake the Chicken: Cover the baking dish with aluminum foil. Bake in the preheated oven for 20 minutes. Remove the foil and continue baking for another 15 minutes, or until the chicken is cooked through and the internal temperature reaches at least 165°F (74°C).
Serve: Once done, remove the chicken from the oven and let it rest for a few minutes. Serve immediately, with gravy if desired.