Clean the onions, put them in a bowl containing water and ½ a glass of wine vinegar, and leave them to soak overnight; this is a little trick to take the edge off the onions’ strong flavour.
The day after, heat the olive oil in a heavy-bottomed frying pan and tip in the thinly sliced onions, after straining them and patting them dry. When they begin to brown, turn up the heat and pour in the wine; when it has evaporated completely, turn the heat down again and season with salt and pepper. Continue cooking over a moderate heat, with the lid on the pan, for about half an hour. Add 1 or 2 spoonfuls of broth if necessary, to keep the onions from burning: they can be “difficult”. Grease a heatproof dish with 2 tablespoons of olive oil, and spread out 1 or 2 spoonfuls of the cooked onions in the bottom. Cover with the toasted bread; pour in 2 ladles of broth, more onions and a good sprinkling of grated pecorino. Cook in a preheated oven, at 180 °C, until the top is well browned. Leave the soup to “rest” for at least 15 minutes before serving.
(1) The curious name given to this soup in Florence comes from the Greek word “Karabos”, meaning a shell-shaped boat. From the idea of the shell it passed on to a soup terrine, and then the soup itself. This recipe is mentioned in a cookery book that dates back to the 1500s, with the name of Carabazada (in those days crushed almonds, cinnamon, sugar and vinegar were added to the onions). And it is in that period, the Renaissance, that it began its journey to France – “exported” by Catherine de’ Medici to the court of her husband, the King of France – where, after a few adaptations, it became the famous “soupe à l’oignon”.