Soupe à l'oignon gratinée — Paris, France

Onion soup is as old as onions themselves, but the version topped with bread and cheese and finished under a grill is associated with Paris's old Les Halles market, where brasseries served it through the night to porters, market gardeners and anyone still out after the bars had closed. Les Halles moved out to Rungis in 1969, and the wholesale market itself is gone from the city centre, but the soup stayed on the menus of the surrounding streets and spread from there to bistros across the city and beyond. The onions are sliced and cooked over low heat for a long time, until they turn a deep golden colour that borders on caramel — this slow step is what gives the soup its flavour, more than any single seasoning. A little flour thickens the base before beef stock and white wine are added; the soup then simmers gently before being ladled into bowls, topped with stale bread and a heavy layer of Gruyère, and passed under a hot grill until the cheese browns and bubbles. Some versions use chicken stock instead of beef, or a mix of the two; others stir a splash of cognac or port into the pot near the end. The bread can be baguette, country loaf, or anything sturdy enough to float without falling apart, and the cheese layer varies in thickness from a light dusting to a dome that spills over the rim of the bowl.