The thinner the grelos are sliced, the faster they will cook. To make preparation easier, roll the leaves up tightly into the shape of a cigar, and slice using a very sharp knife. If the grelos are not in season, they might be a little tough so you can blanch them prior to adding them to the soup. Also, if you want to strengthen the chouriço flavor of the soup, you can boil the sausage links with the potatoes, onion and garlic and remove just before blending. If you are unable to find <em>grelos</em>, you could replace them with turnip greens.