To make slicing the pork chop easier, freeze it for 15 minutes. We prefer the distinctive flavor of Chinese black vinegar; look for it in Asian supermarkets. If you can't find it, a combination of red wine vinegar and balsamic vinegar approximates its flavor. This soup is very spicy. For a less spicy soup, omit the chili oil altogether or add only 1 teaspoon. Recipe stolen from the PBS cooking program America's Test Kitchen. This is an excellent soup.