Soupe a l'Oignon Gratinee

Soupe a l'oignon is a French onion soup whose recognizable modern form — onions, broth, crouton, and melted cheese gratineed on top — took shape in Paris around the eighteenth century, though onion soups in various forms reach back much further. The version that became a bistro standard uses slowly caramelized onions, a good beef broth, a slice of toasted bread, and a thick layer of Gruyère browned under the broiler. The dish is associated with the Parisian night: served at Les Halles market stalls through the small hours, it was long the sustenance of the workers and market porters of the central markets.

A bowl of onion soup arrives with a thick, browned raft of cheese stretched over the top, the bread beneath soaked through with broth. The first spoon breaks the cheese and sinks into the dark, sweet onion broth; the onions have melted to a soft, almost jammy sweetness from the long caramelization. The broth is deep and savory, the cheese adds a salty richness, and the soaked bread gives body. Eaten very hot.

French onion soup rests on the slow caramelization of the onions. Sliced onions are cooked in butter over low heat for the better part of an hour, during which their starches and sugars break down and undergo Maillard browning and caramelization; the cell walls collapse, the moisture evaporates, and the natural sugars concentrate and brown into the deep, sweet, jammy character that defines the soup. Beef broth is added and simmered; the gelatin from the broth's bones gives body. The gratinee — the cheese crust — is built by floating a slice of toasted bread on the surface, covering it thickly with grated Gruyère, and placing the bowl under a broiler. The cheese melts, browns by Maillard reaction, and forms a sealed raft that keeps the soup beneath very hot; the bread absorbs broth from below and cheese from above, softening into a rich, almost custardy layer.