I had seen these sandal-sole sized presumably fried things around Mexico City last week. Almost black, I guessed they were made from blue corn.
According to Wikipedia:
Tlacoyo [tɬaˈkoʝo] is an oval-shaped fried or toasted cake made of masa. Somewhat torpedo-shaped, they are fatter than fresh corn tortillas and stuffed with cooked and ground beans, cheese, fava beans, chicharron or other ingredients. Tlacoyos can be served as an accompaniment to soups and stews or as appetizers for celebrations. Most traditional tlacoyos do not have lard or salt in the masa, and if not eaten soon after they are cooked, they become very tough and dry, even if reheated. On Mexican markets, vendors keep their tlacoyos warm by putting them in a covered basket, in order to keep them moist for a longer time. This dish is similar to the Salvadoran pupusa.
That is what it looked like, with the addition of cheese and lettuce. I added a salsa verde. Inside the chewy bread-like substance was a white creamy cheese.