The most celebrated soup in French cuisine — an intensely flavoured broth built on slow-caramelised onions cooked until soft, sweet, and dark golden, then topped with a thick slice of toasted bread and a blanket of melted Gruyère cheese, gratinéed until bubbling; the classic bistro dish of Paris.
Caramelisation — the essential step
French onion soup is defined by one process: the slow caramelisation of onions. Onions are cooked in butter over low-to-medium heat, stirring regularly, for 45–60 minutes until they turn soft, golden-brown, and intensely sweet. During this time, the onions lose most of their volume and develop complex flavour compounds. Rushing this step with high heat produces inferior results — the deep sweetness and colour can only be achieved slowly.
The soup
Caramelised onions are deglazed with wine or brandy, then simmered in good beef stock (or dark vegetable stock) with thyme and bay. The quality of the stock matters enormously — weak stock produces weak soup. The broth should be deeply savoury and faintly sweet.
Gratinéed finish
The defining feature of the classic soupe à l’oignon gratinée is its finish: soup is poured into oven-proof bowls, a round of toasted baguette is floated on top, and the whole surface is covered in grated Gruyère or Comté cheese. The bowls go under a hot grill (broiler) or into a very hot oven until the cheese melts and bubbles into a browned, chewy cap. Eating through the cheese cap into the soup below is one of the pleasures of bistro dining.
Origins
Versions of onion soup appear in French cooking from at least the 18th century. The gratinéed version became associated with Les Halles, the central Paris wholesale food market, as a workers’ meal eaten by market porters through the night and into the early morning.
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