Place the turkey, skin side up, in a slow cooker. Brush with butter. Sprinkle with parsley, tarragon, salt if desired and pepper. Top with mushrooms. Pour wine or broth over all.
Cover and cook on low for 7-8 hours.
Remove turkey and keep warm. Skim fat from cooking juices. In a saucepan, combine cornstarch and water until smooth.
Gradually add cooking juices. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve with the turkey.