Combine sliced onions, butter, sugar, and salt in your slow cooker.
Cover and cook on high for 7 hours, or until the onions are softened and caramelized.
Add the beef broth, sherry, thyme sprigs, and pepper to taste.
Cover again and cook on high for an additional hour.
When ready to serve, ladle the soup into bowls. Top each bowl with a slice of bread (or croutons) and Gruyere cheese.
Pro-tip: For an extra melty cheese topping, cook for an additional 15 minutes after adding the cheese.
