This is an unusual marinade from the Mexican state of Baja that includes soy sauce, an ingredient you might not think of as Mexican, but which has been used on that country's Pacific coast since the 1500s.
Mix all the marinade ingredients together and submerge the skirt steak in it, making sure every bit is coated with the marinade. Let this sit in the fridge for at least 1 hour, and up to overnight, although the longer you marinate the saltier the meat will get. About 4 hours is a nice amount of time.
Mix the chopped onion with the lime juice and a pinch of salt. let this sit at room temperature while you marinate the meat.
When you are ready to grill, get your fire very hot. While this is happening, heat up your tortillas on a comal or heavy skillet; keep them warm in a tortilla warmer or wrapped in kitchen towels.
Remove the steak from the marinade and pat the meat dry with paper towels. Make sure it's dry. Rub oil all over the meat.
Sear the skirt steak on the grill for only about 2 minutes per side, tops. Let it rest for a few minutes before slicing. My trick is to cut the skirt steak across the grain first, then keep chopping until you have little bite-sized pieces. Doing this ensures that you will have tender meat. Serve the skirt steak with cilantro and the quick pickled onions, plus some salsa.