This simple soup, which we came to think of as the hot-and-sour soup of Mexico, was cooked for us by Chabela Cortés García in Hidalgo. The sourness comes from xoconostle--sour prickly pears. They're tangy and succulent, sort of like small lemony melons. Sometimes you'll find them at farmers' markets, but the sweet prickly pears are far more common north of the border. (They look almost identical, so be sure to ask; the sour type have all their seeds in the middle, and seeds in the sweet ones are spread through the fruit.) You can use dried prickly pear instead, but it's not nearly as tangy, so add a bit of distilled white vinegar to make it taste closer to the fresh.