In your largest thick-bottomed pot over medium heat combine the olive oil, celery, garlic, carrot, and red onion. Cook for 10 -15 minutes sweating the vegetables, but avoid any browning. Stir in the tomatoes and red pepper flakes, and simmer for another 10 minutes or so, long enough for the tomatoes to thicken up a bit. Stir in the cavolo nero, 3 cups of the beans, and 8 cups / 2 liters water. Bring to a boil, reduce the heat, and simmer until the greens are tender, about 15 minutes.

In the meantime, mash or puree the remaining beans with a generous splash of water - until smooth. Tear the bread into bite-sized chunks. Stir both the beans and bread into the soup. Simmer, stirring occasionally, until the bread breaks down and the soup thickens, 20 - 30 minutes. Stir in the salt, taste and add more if needed. Stir in the lemon zest.

Serve immediately, or cool and refrigerate overnight. Serve reheated, or "ribollita" meaning reboiled, the next day ladled into bowls. Finish each serving with a drizzle of olive oil and some chopped olives.

Makes a large pot of soup - enough for 10 servings.

Prep time: 20 min - Cook time: 60 min