When the Spaniards arrived in Mexico the Aztecs or Mexica as they called themselves were already using the tortilla known in Náhuatl as tlaxcalli.  Beef was not on the menu at the time until later as the Mexica did not have horses nor cattle.  The tortilla was a staple as bread was to the Europeans.  Corn or maize was an integral part of Mexica diet and religion.  Corn was originally a wild grass and was developed into a usable and tasty crop known to the Mexica as achichilyauitl or maíz rojo. 

Sopa de Tortilla