For over a century this soup was served at 3 a.m. to the porters and butchers of Paris's old central food market, and after-hours revelers used to detour there just to eat alongside them.
Onions are caramelized slowly in butter, simmered in beef stock, then ladled into a bowl over toasted bread, topped with melted Gruyère, and broiled until bubbling. Cafés near the old Les Halles market still serve it as a late-night, warm-you-up classic.