1 lb flank steak
Things to cook the steak with
1 large onion, thinly sliced
3 heaping teaspoons minced garlic
Juice of 1 lime
1 tablespoon salt
Olive Oil to coat the pan

Cook the steak. Simmering for a while works, stewing for a while works too. You'll probably want other things to flavor it. Usually takes a couple of hours. See "ropa vieja" recipe for more details. Shred the meat using your hands and/or a fork.
Add salt, garlic, half of the sliced onions, and the lime juice.
Coat a frying pan with olive oil and add mixture to the cold pan without heating the oil.
On medium heat stir the meat constantly, adding additional onions and more oil gradually as needed.
Once the onions are soft and the meat cooked, raise the heat and sear the meat until it’s slightly brown and crispy.
Serve with white rice and black beans, or plantains, or all three!

Source: http://cubanfood.blogspot.com/2009/05/esthers-vaca-frita.html and
http://agozarnyc.com/blog/2012/08/09/vaca-frita-authentic-cuban-food-with-history/