Heat oil in large skillet over medium heat until hot. Add onion; cook about 4 minutes or until tender, stirring occasionally.
In 3 1/2- to 6-quart slow cooker, combine onion, celery, carrots, turkey, wild rice, tarragon and pepper; mix well. Pour broth over top.
Cover; cook on low setting for 6 to 8 hours.
About 25 minutes before serving, in small bowl, blend milk and flour until smooth. Gradually stir into soup. Add peas and sherry; mix well. Cover; cook on low setting an additional 20 minutes or until sauce has thickened.