Preheat oven: Preheat your oven to 375°F and grease a 9x13 baking dish with oil or non-stick cooking spray.
Sauté vegetables: In a small skillet, melt 1 tablespoon butter over medium heat. Add ½ cup diced onion and ½ cup diced celery. Sauté for about 5 minutes, or until the vegetables are tender and slightly caramelized. Remove from heat and set aside.
Prepare stuffing mix: Heat 1½ cups chicken broth in the microwave for 2 minutes or bring to a low boil on the stovetop. In a large bowl, combine the 6 oz stuffing mix, heated broth, 1 tablespoon remaining butter, and ½ cup milk. Stir in the sautéed onions and celery, and ½ cup dried cranberries. Mix until well combined and set aside.
Butterfly the chicken: Trim any fat from the 4 chicken breasts. Carefully slice each breast ¾ of the way through the thickest part, being sure not to cut all the way through.
Stuff chicken: Divide the stuffing mixture into 4 equal portions. Stuff each chicken breast with ¼ of the stuffing mixture and place the stuffed breasts into the greased baking dish.
Season chicken: In a small bowl, mix ½ teaspoon salt, ½ teaspoon pepper, and ¼ teaspoon onion powder. Sprinkle the seasoning evenly over the stuffed chicken breasts.
Bake covered: Cover the baking dish with foil and bake for 20 minutes. After that, remove the foil and bake for another 15 minutes, or until the chicken's internal temperature reaches 165°F.
Serve: Remove from the oven and serve immediately with gravy, if desired.