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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.
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