In a bowl, add together 1/2 of chopped onion, lime juice, lemon juice, apple cider vinegar, soy sauce, garlic, salt, chili powder, cumin and cayenne pepper. Mix well.
Add flank steak to the bowl and try to cover as much of the steak as you can in the liquid. Cover with plastic wrap and refrigerate for 1-3 hours in the refrigerator.
Push saute on the Instant Pot when it’s time to remove the flank steak from refrigerator after 1-3 hours.
While the Instant Pot is heating up, cut flank steak into bite-sized pieces.
Once pot is hot, add oil and remaining onion and saute until the onions become translucent, about 3 minutes. Once done, turn Instant Pot off by pushing cancel.
Add chopped flank steak and 1 cup marinade to the pot and close the lid. Press the meat/stew button and adjust time to 8 minutes.
When the timer goes off, allow for a natural pressure release of 10 minutes. Once that is up, release any remaining pressure.
Remove steak from pot and place on flour or corn tortilla. Top with sour cream, cilantro, avocado and some hot sauce (if desired).