Soak the beans overnight as suggested on page 6, or use the Quick-Soak Method on page 6. Drain, discarding any soaking liquid.
In a medium pot, bring the beans and stock to a boil, skimming off the froth that rises to the top. Add the garlic and oregano. Stir and turn the heat down to low. Cover partially and simmer gently for 40 to 60 minutes, or until the beans are tender. (Older beans will take longer to cook.) Crush the garlic clove against the side of the pot and mix well.
Put the oil in a large pan and set over medium-high heat. When hot, add the onions and potato. Stir and cook for 4 to 5 minutes so there is a little bit of browning. Add the chard and parsley. Stir for about 1 1/2 minutes, or until the chard has wilted. Now add the cooked beans and their liquid and bring to a boil. Cover partially, turn the heat down to low, and simmer gently for 30 minutes, stirring now and then. Mash some of the beans and the potato pieces against the sides of the pan. Taste for salt; you will probably need to add some even if your stock was salted. Mix well.
Ladle into soup plates and dribble a teaspoon of extra-virgin olive oil over each serving.