Spray 3 1/2-4 quart slow cooker with nonstick cooking spray. Place onion and celery in slow cooker. Top with turkey breast half. Pour 1 jar of gravy and sherry over top. Top with dry stuffing.
Cover; cook on low setting for 6 to 7 hours.
About 15 minutes before serving, remove turkey from slow cooker; place on cutting board. Stir stuffing and cooking juices until mixed. Cover; let stand 10 minutes. Heat remaining jar of gravy in small saucepan. Cut turkey into slices. Serve turkey and stuffing with gravy.