Ribollita—literally 're-boiled'—is the signature bread soup of Tuscany, a thick, hearty, almost stew-like preparation of cannellini beans, cavolo nero (Tuscan black kale), stale bread, and vegetables that is cooked one day, then reheated ('re-boiled') the next, during which it transforms from a soup into something approaching a savoury bread pudding of extraordinary depth and comfort. The dish is the apex of Tuscan cucina povera: every ingredient is humble, every technique is simple, yet the result is one of Italy's most satisfying dishes. The canonical method begins with a soffritto of onion, carrot, celery, and garlic in olive oil, to which are added tomato, potatoes, zucchini, chard, and—essentially—cannellini beans (about a third of which are puréed to thicken the soup). Cavolo nero, the Tuscan lacinato kale with its dark, crinkled leaves and distinctive mineral-sweet flavour, is the indispensable green. The soup simmers until all the vegetables are tender and the flavours unified. Here comes the critical step: slices of stale Tuscan bread (pane sciocco—the unsalted bread that is Tuscany's signature loaf) are layered into the soup, which is left to rest overnight. The next day, the soup is 'ribollita'—reheated slowly, during which the bread absorbs the liquid and breaks down, the beans dissolve further, and the whole mass thickens into a consistency dense enough to eat with a fork. A generous drizzle of the best Tuscan extra-virgin olive oil (new-harvest, if possible—peppery and green) and a grinding of black pepper finish each bowl. Ribollita is Tuscany's answer to the question of what to do with leftover bread and vegetables—a question whose answer has sustained Tuscan farming families for centuries.
Cannellini beans (some puréed for thickening). Cavolo nero is essential—no substitute. Layer with stale unsalted Tuscan bread. Cook one day, reheat the next (the 'ribollita' step is mandatory). Finish with Tuscan olive oil. Should be thick enough to eat with a fork.
Use genuine pane toscano (unsalted bread)—salted bread changes the balance. The soup should be very thick after reheating—add more bread if too liquid. New-harvest (novello) Tuscan olive oil, poured generously, transforms the dish. Some Tuscans bake the ribollita in the oven after reheating to form a crust. Red onion from Tropea is traditionally preferred.
Skipping the overnight rest and reheating (not ribollita without the re-boiling). Using fresh bread instead of stale. Omitting the cavolo nero. Making it too thin (should be very thick). Using insufficient olive oil at serving.
Pellegrino Artusi, La Scienza in Cucina; Giuliano Bugialli, The Fine Art of Italian Cooking