In a large resealable bag, combine the steak, olive oil, garlic, oregano, sazon seasoning, onions, vinegar, beef stock, and salt. Seal and shake to mix. Refrigerate for at least 4 hours, or up to a couple of days.
When you are ready to cook, dump the entire contents of the bag into a large deep skillet. Bring to a boil, reduce heat to low, cover and simmer until the beef is fork tender, about 40 minutes.
