STEAK MARINADE: Combine the ingredients for the marinade in a large shallow bowl except for the steak. Give it a taste and check for seasonings, adjust to preference. Coat the steak with the marinade and let sit covered in the refrigerator for at least 3-24 hours, covered. Remove from the refrigerator 30 minute prior to cooking.
COOK THE STEAK: Set a large cast iron skillet or pan over high heat and let heat up for a couple of minutes. Add the steak to the pan and cook on each side for about 3-6 minutes or longer depending on the thickness and your preference for doneness. Feel for desired firmness. 4 minutes/side will yield a medium doneness. If the pan becomes too hot, reduce the heat to medium-high.
LET REST: Remove the steak from the pan and allow to rest for several minutes. Slice the meat against the grain into thin slices. If dicing, I like to do so at an angle so the meat cubes are tender.
ASSEMBLY: Grab a warm tortilla and spread with my quick guac (recipe in notes), top with the steak, diced onions, cilantro, and squeeze on a little lime! Enjoy!