Onion is the main ingredient in French onion soup.
The dish is built around sliced onions that are slowly cooked in butter or oil until soft and deeply golden. This long browning step concentrates their natural sugars and creates the soup's characteristic sweet, savory flavor. The onions are then simmered in stock or water, commonly with beef stock, before the finished soup is topped with bread and melted cheese.
Onion soups have been known in France since at least the medieval period, when onions were inexpensive and widely available. A 1393 French recipe included onion soup, while François Pierre de La Varenne described onion-soup preparations in his 1651 cookbook. The dish was often associated with humble cooking because its core ingredient was affordable.
The familiar gratinéed version became especially associated with Paris's Les Halles market in the mid-19th century. Gruyère, Comté, or Emmental may be used for the cheese topping. A common mix-up is treating the bread, cheese, or stock as the defining ingredient; those vary, but onions are essential to the soup's identity.