- Place onions, butter, sugar and salt in the bottom of a 6-quart slow cooker. Cover and cook for 7 hours on high, or until onions are soft and brown.
- Add beef broth, sherry, thyme sprigs and pepper to taste. Cover, and cook on high for another hour.
- To serve, add sliced bread (or croutons) across the surface of the soup. Add sliced Gruyere cheese and serve.
