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.
