In a 6-quart slow cooker, combine onions, butter, sugar, and salt. Cook on high for 7 hours or until onions are tender and have a rich brown hue.
Introduce beef broth, sherry, thyme, and season with pepper. Mix well, cover, and let it simmer on high for another hour.
At serving time, float slices of bread or croutons atop the soup. Crown with slices of Gruyere cheese.
Optional: For a more molten cheese layer, cook an additional 15 minutes.
Indulge in this classic delight, made easier and just as flavorful with a slow cooker!
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