These large, thick tortillas are called "tlayudas" and they're a Oaxaca specialty that you're unlikely to find elsewhere in the country. They're prepared by spreading pork fat and bean paste on the tortilla, then the Oaxaca string cheese called quesillo is added in, plus some shredded lettuce or cabbage to add a little crunch. It's folded over and toasted on a grill until it's crispy and the cheese inside melts, and served with your choice of meat. If you want yours without the pork fat, just ask for it "sin aciento."
Tlayudas are served in many places in Oaxaca. In restaurants they're usually served open-faced, which is perhaps more attractive, but when you have it folded over like this, the cheese melts more and combined with the crisp tortilla, it's really delicious.
Tlayudas Libres opens at 9 pm nightly and closes at 3 or 4 am. They have grills set up on the street so you can watch how they're prepared. This is a popular late-night stop after an evening of partying.