Combine steak, onions, beef stock, olive oil, vinegar, garlic, Sazón seasoning, salt, and oregano in a large resealable plastic bag; toss to coat steak with marinade. Squeeze out excess air and seal the bag. Marinate in the refrigerator at least 4 hours up to 2 days.

Transfer steak and marinade to a large deep skillet; bring to a boil. Reduce heat to low; cover and simmer until steak is fork-tender, about 40 minutes.