Discard any beans that have floated to the surface of the water. Drain and rinse the beans under cold running water, put them in a large pot, and add the broth and the potato. Cover the pot partially and bring the broth to a gentle boil over medium heat. Reduce the heat to low and simmer gently, stirring occasionally, until the beans are tender, 45 minutes to 1 hour.

While the beans are cooking, prepare the soup base. Heat the oil in a medium-size saucepan over medium heat. Add the garlic and the parsley and cook, stirring, until garlic begins to color, about 1 minute. Add the tomates and bring to a gentle boil. Season with salt and pepper, and reduce the heat to medium low. Simmer until sauce has a medium-thick consistency, 20 to 25 minutes.

Puree the beans, potato, and all the broth in a food processor until smooth, and return puree to the pot. (You might have to puree the soup in batches.) Add the tomato sauce to the beans and cook, over medium heat, 4 to 5 minutes longer. Serve hot or at room temperatue and drizzele a few drops of olive oil over each serving.

For a heartier soup, add some pastina, or toast some bread slices in the oven, place them in the bowls, and top with the soup.