In a bowl mix together red wine vinegar, olive oil, salt, oregano, cumin and garlic cloves.
Place flank steak in a large freezer bag or a large rectangular glass baking dish. The bag tends to work better but if you use a dish, flip it over a few times while marinating.
Pour marinade in the bag or over it and refrigerate for about 8-10 hours.
Prior to cooking, slice steak into ¼" to ½" slices on a bias against the grain of the meat.
Heat olive oil in skillet on medium heat. Add onions; toss until translucent. Do not brown the onions.
Add chicken stock and bring to a boil for about 3-4 minutes. Continue to simmer until all the liquid has cooked off and been absorbed by the onions.
Transfer onion mixture to a bowl.
In another skillet, place the remaining 1 Tbsp. of olive oil and quickly sauté the pieces of flank steak until medium or well done.
Place on a serving platter and spoon onion mixture over the beef.
Serve with arroz con gandules and maduros.