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, chuck the entire contents of the bag into a large deep frying pan. Bring to the boil, reduce heat to low, cover and simmer until the beef is fork tender, about 40 minutes.
Sazón seasoning is used throughout Latin America and the Caribbean. Look for it in Caribbean markets or in larger supermarkets. Goya is a popular brand.
Water can be used in place of beef stock if desired.