"Don Charly's," located on Gyeongnidan-gil, Itaewon, is reputed to serve the most authentic Mexican cuisine (or Tex-Mex, rather) in the city. "Don Charly's" specializes in a variety of tacos and tortas. Even though the place is tiny, the food is a real deal and the chef, Carlos from Mexico D.F, does not gimp out on the ingredients!
*This place has been moved to: http://www.trazy.com/spot/1869/