In a skillet over medium heat, cook onion in oil for about 4 minutes, until tender. Combine onion with turkey, celery, carrots, rice and seasonings in a slow cooker; stir in broth. Cover and cook on low setting for 6 to 8 hours. Mix evaporated milk and flour; stir into soup along with peas and sherry, if using. Cover and cook on low setting for about 20 minutes, until thickened.