In a 5- to 6 1/2-quart slow cooker, mix onions and melted margarine or butter.
Cover and cook on High 30-35 minutes, until onions begin to brown slightly around the edges.
Mix flour, Worcestershire sauce, sugar and pepper. Stir flour mixture and broth into onions. Cover and cook on Low 7-9 hours or High 3-4 hours, until onions are very tender.
Prepare cheesy broiled French bread and place one slice on each bowl of soup. Serve immediately.