Tuscany — ribollita is documented from the medieval period as the soup of Florentine contadini (peasant farmers). The saltless bread tradition of Tuscany (pane sciapo, developed historically to save salt, a taxed commodity) is essential to the preparation. The cavolo nero cultivation is specific to the Tuscan winter garden.
Ribollita ('reboiled') is Tuscany's most celebrated soup — a preparation that begins as a minestrone di cavolo nero (Tuscan kale soup with cannellini beans, cavolo nero, and bread), is allowed to cool and solidify overnight into a thick, bread-dense mass, then reheated (reboiled) the following day. The reheating and stirring of the solidified soup produces a completely different, more unified, more intensely flavoured preparation than the original soup. Without the overnight rest and the ribollita — the reboiling — it is not ribollita; it is simply minestrone. The cavolo nero (Tuscan black kale, Lacinato kale in American parlance) is the defining vegetable.
Ribollita on the second day is a different preparation from the first — where the minestrone was liquid and varied in texture, the ribollita is thick, unified, and deeply flavoured. The bread has dissolved into the broth; the cavolo nero has become silky and dark; the beans provide creaminess. A thread of raw DOP Tuscan olive oil over the top is the finishing touch that elevates the entire preparation. It is the taste of Tuscany in January.
Day 1 (minestrone): make a base of soffritto (onion, carrot, celery, garlic, rosemary) in olive oil with a piece of guanciale or lard. Add cavolo nero (stripped from stems, roughly chopped); cook down 10 minutes. Add cannellini beans (cooked); add their cooking water and additional water. Add cubed stale Tuscan bread (saltless pane sciapo — essential; salted bread changes the preparation). Simmer 30-40 minutes. Season; cool; refrigerate overnight. Day 2 (ribollita): remove fat that has solidified on top. Slowly reheat while stirring and breaking down the bread — the soup becomes thick and uniform. Drizzle excellent DOP Tuscan olive oil over each serving.
Cavolo nero (Lacinato kale, Tuscan black kale, dinosaur kale) is now widely available in UK supermarkets and farmers' markets. Tuscan saltless bread (pane sciapo) can be approximated by any good sourdough or white bread that has been left to go stale without refrigeration. The ribollita improves each day it is reheated — many Tuscan cooks make it on Wednesday for Saturday lunch.
Not letting it rest overnight — ribollita made and served the same day is minestrone, not ribollita. Using salted bread — Tuscan bread (pane sciapo, saltless) is essential; salted bread produces a different texture and makes the soup too salty on reheating. Insufficient cavolo nero — the cavolo nero is the vegetable that makes this specifically Tuscan; Swiss chard or regular kale are substitutes that produce a different flavour.
Marcella Hazan, Essentials of Classic Italian Cooking; Slow Food Editore, Toscana in Cucina