Melt butter in a skillet over medium heat. Add onion; cook for 15 to 20 minutes, or until soft. Stir in sugar and salt; continue to cook and stir until golden. Add flour; mix well. Combine onion mixture, broth and wine or broth in a 4-quart slow cooker. Cover and cook on high setting for 3 to 4 hours. Ladle soup into oven-proof bowls. Top with bread slices; sprinkle with cheeses. Broil until cheese is bubbly and melted.