Cut the beef into 5x5 size if they are bigger than that.
Place saran wrap over one piece of beef and tenderize with a tenderizer well on both sides. I pound mine until I see the beef has become visibly thinner. Do this one piece at a time and place in a large bowl as you go.
Rinse the beef with water and drain.
Pour in the red wine vinegar and massage into the beef for about 30 seconds.
Rinse again with water and drain. The beef with feel a bit slippery after this.
Add garlic, salt, pepper, white vinegar, soy sauce, adobo and oregano. Mix well into the beef and store for at least 4 hours but overnight is preferred.
If you are like me and like to get things done and out the way you can now slice the onions and store for the next day or do so right before you start to cook your steak.
Once ready to cook, heat oil in large skillet over medium high heat.
Add beef and cover. (RESERVE the juices from the seasoning and add 1/2 cup water to it.)
Now you are going to give the beef some color, turning once before lowering the heat to simmer.
After about 5 minutes, add the red wine or red cooking wine. Cover.
This beef will cook for about 1 hour and you'll be checking on it periodically making sure it doesn't dry out and adding bits of that reserved liquid and turning the pieces over a couple of times. Add more water to the liquid if needed.
Once the beef is tender and still has liquid add the onions over the beef and cover. You are now going to allow the onions to sweat over the beef and subsequently will begin to caramelize.
Once onions are becoming tender, stir around in the skillet and keep uncovered to make sure are not over cooking the onions. You'll see them becoming caramelized and the beef sitting in more oil than water and getting some color. This is when you are done and the beef is ready to enjoy.