In 1972, I spent the winter in San Cristóbal, a small town in Mexico's Chiapas province, near the Guatemalan border. At sundown, a tantalizing smell would waft along the street. A neighbor had set up a charcoal grill outside her front door where she was making tacos. I'd run down with a plate and join the line. The tacos were soft tortillas stuffed with spicy braised pork and topped with pickled red onions. They were so good that later that year, on a visit home to England, I decided to make this dish for my parents. I had the spices and the pork; all I needed were tortillas. I found them at Harrods...