Bread, Potato and Arugula Soup (Zuppa di Patate, Rucola, e Pane)

The ideal arugula to use for this soup is the tougher, wilder kind; the fragile hydroponic type is too delicate for this dish. Serves: 8 Ingredients ¾ pound medium-size potatoes (about 2 potatoes), peeled and cut into ½-inch-thick slices 1 pound arugula (2 to 3 large bunches), preferably not hydroponic or “baby” ½ cup dices (½-inch) day-old…