Place the chicken, potatoes, carrots and celery into a 6-quart slow cooker.
Stir the soup, water, thyme and black pepper in a small bowl. Pour the soup mixture over the chicken and vegetables.
Cover and cook on LOW for 7 to 8 hours* or until the chicken is cooked through.
Stir the baking mix and milk in a medium bowl. Drop the batter by spoonfuls onto the chicken mixture. Increase the heat to HIGH. Tilt the lid to vent and cook for 30 minutes or until the dumplings are cooked in the center. *Or on HIGH for 4 to 5 hours.