Preheat the oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish with olive oil or nonstick spray.

In a medium saucepan, bring the chicken broth and butter to a simmer.

Stir in the stuffing mix, remove from heat, cover, and let it sit for 5 minutes.

Fluff the stuffing with a fork and stir in onion, celery, or dried cranberries if using.

Pat the chicken breasts dry with paper towels.

Carefully cut a deep pocket into the side of each chicken breast without slicing all the way through.

Season the outside and inside of each chicken breast with salt, pepper, and onion powder.

Spoon a generous amount of stuffing into each chicken breast pocket.

Secure the openings with toothpicks if needed and place the stuffed chicken breasts into the prepared baking dish.

Pour the milk or additional chicken broth around the chicken in the baking dish if using.

Cover the dish with foil and bake for 25–30 minutes.

Remove the foil and continue baking for 10–15 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and is lightly golden.

Let the chicken rest for 5 minutes before serving.

Serve warm with gravy or pan drippings, alongside vegetables or mashed potatoes.