In a large bowl, mix together vinegar, adobo, pepper and oregano. Add steak and onions to oil mixture, tossing to combine. Cover beef; transfer to refrigerator. Marinate at least 1 hr., or up to 12 hrs.
Heat oil in large skillet over medium-high heat. Add beef, onions, and marinade to the pan; cover the pan and cook, stirring occasionally, until onions turn translucent and beef is cooked through, about 10 minutes.
Evenly divide meat among serving dishes; top with onions and drizzle with sauce.
If you can’t find cube steaks in your grocery store, you can swap in 2 lbs. top round steaks, instead. Simply cut the beef into six 5-oz pieces and working with one piece at a time, place between two sheets of plastic wrap. Using the smooth side of a mallet, or a rolling pin, pound beef to ¼” thickness, taking care not to tear the beef, and proceed with step 1.