In a 6-quart slow cooker, combine onions, butter, sugar, and salt. Cover and cook on high for 7 hours, or until the onions are soft and browned.
Add beef broth, sherry, thyme sprigs, and pepper to your preference. Cover and cook on high for an additional hour.
To serve, place sliced bread (or croutons) on the surface of the soup and top with sliced Gruyere cheese.
Enjoy this effortless, slow-cooked version of French onion soup, bringing all the gourmet flavors of the restaurant classic right into your home kitchen.