In a pot submerge the beans with salted, cold water (2 inches of water should surpass the beans). Boil until tender, adding more water if necessary.

Sauté the onion, carrot, celery, and parsley in olive oil in a heavy bottomed pot. When the onion has become translucent, add the tomato paste and the liquid from the beans. Then add the cabbage, beet greens, and potatoes.

Stir in the beans and season to taste with salt, pepper, and a sprig of thyme. Simmer until the potatoes are done. Remove the thyme. Take a deep oven-proof serving dish and layer it with soup, alternating with sliced bread. 

The soup can be enjoyed immediately as bread soup, or you can allow it to rest overnight then re-heat it and enjoy it as Ribollita.