Coat inside of a 4-quart electric slow cooker with cooking spray. Add stuffing mix, onion, celery, and cranberries. Combine broth and melted butter. Pour over stuffing, and stir gently.
Remove string from turkey breast. Rinse turkey breast. Place turkey in slow cooker on top of stuffing. Combine salt, pepper, and thyme; sprinkle over turkey. Cover and cook on HIGH 1 hour. Reduce to LOW, and cook 5 to 6 hours or until a meat thermometer inserted in turkey registers 170°.
Remove turkey to a serving platter. Stir stuffing gently in slow cooker; cover and let stand 3 to 4 minutes. Prepare gravy. Spoon stuffing around turkey on platter.
Note: Some turkey breasts come with a gravy packet; some don't. We liked the flavor and color of gravy using a (0.88-ounce) package of turkey gravy mix (we tested with French's).