Heat the oil in a large soup pot. Saute the onions over moderate heat until translucent, 3 or 4 minutes. Add the garlic and saute until the onion is lightly golden, another 3 or 4 minutes. Add the remaining ingredients and bring to a simmer. Mash some of the beans with a potato-masher, just enough to thicken the liquid base of the soup. Cover and simmer gently but steadily for 10 minutes.