Pick them over, discarding any misshapen beans or stones. Rinse well, place in a bowl, and add water to cover generously. Let soak overnight. Drain, place in a large saucepan and add the water. Bring to a boil over high heat and add the sage, rosemary and garlic. Reduce the heat to low, cover partially, and simmer very gently until the beans are very soft, 1 ½ to 2 hours, depending on the age of the beans.
Make the soup
In a frying pan over medium heat, warm the olive oil. Add the onion and sauté until fragrant, (about 3 minutes). Add the chopped sage and piece of pancetta and sauté until the pancetta is golden, (about 7 minutes). Add the chopped garlic and sauté until it begins to colour, (about 2 minutes). Remove the cooked pancetta and discard. Reduce the heat to low, stir in the tomatoes (or puree) and simmer gently, stirring occasionally, until the tomatoes are soft, (about 10 minutes).
Remove the herb sprig and garlic from the beans and discard. Using a slotted spoon, transfer the beans in batches to a food mill fitted with the medium disk, held over a soup pot, and purée directly in to the pot, adding cooking liquid as needed to facilitate the puréeing. Add 2 c of the cooking liquid to the broth or water and set aside. (If you’re using the tomato purée instead of the tomatoes, reduce the amount of cooking liquid added, accordingly). Using the food mill, purée the tomato/onion mixture, letting it pass directly into the soup pot. Stir in the broth or water and place the pot over medium-low heat.
Add the rosemary sprig, season with salt and pepper and cook uncovered, stirring occasionally, until the soup has thickened and the flavours are blended, (about 30 minutes). Discard the rosemary sprig.
Transfer the soup to a warmed tureen. Sprinkle with the parsley and serve at once.