Wash and chop the onion and celery into small pieces.
In a large bowl, combine the dry stuffing mix, melted butter, chopped onion, chopped celery, and poultry seasoning. Mix until moistened.
Gradually pour the chicken broth into the stuffing mixture and stir well.
Spread the stuffing mixture evenly in the bottom of your slow cooker.
Season the chicken breasts with salt and pepper on both sides.
Arrange the seasoned chicken breasts on top of the stuffing in a single layer.
Pour the cream of chicken soup on top of the chicken breasts, spreading evenly.
Cover the slow cooker and cook on low for 6-8 hours, or on high for 3-4 hours, until chicken is cooked through (165°F/74°C) and stuffing is tender.
Let the chicken and stuffing rest for 10 minutes before serving.