The magic
The trick to getting this soup right is that the onions caramelise – they must cook for at least 35-40 minutes over a low to moderate heat. Only when the onions are soft enough to crush easily between finger and thumb can you add the flour and stock.
The twist
Contemporary versions can involve roasting the onions in the oven to give a more concentrated sweetness. Red onions produce the sweetest version of all, so you might like to add thyme and bay as a balance. Parmesan in place of Gruyère produces a less fatty but just as tasty crust.