By THÉRÈSE MARGOLIS I have always been a squeamish eater, and frankly the idea of pork belly stewed in a broth of hominy and chili peppers topped with sliced radishes and lettuce never really appealed to me. So when my friend Enrique Castillo Pesado invited me to lunch at one of Mexico’s 20-plus Potzollcalli restaurants, my immediate response was, “Thanks,