Season and cook the beef to your liking. Chop or shred the beef. Warm the tortillas on a hot cast iron griddle and put in tortilla warmer or wrap in a towel to keep warm. Chop the onion and the cilantro and mix together. Quarter the limes.
Putting them together: Put the chopped beef in a warm corn tortilla add the guacamole and onions/cilantro. Squeeze a little lime over the top and enjoy.