In 8-inch skillet, heat oil over medium heat. Add onion; cook about 4 minutes, stirring occasionally, until tender.
Spray 1 1/2- to 2-quart slow cooker with cooking spray. Place onion, celery, carrots, turkey, wild rice, tarragon and pepper in cooker. Pour broth over top.
Cover; cook on Low heat setting 6 to 8 hours, stirring in peas for last 15 minutes of cooking.
In small bowl, mix milk and flour; stir into soup. Cover; cook about 10 minutes or until thickened. Stir in sherry before serving.