In a large pot, cover the flank steak (you may need to cut it in half or even quarters), bay leaf, and quartered onion with enough water to cover the meat by an inch.

Bring to a boil, and then simmer over low heat for 20 minutes.

While the beef is cooking, in a medium skillet over medium-low heat, heat the butter. Gently cook the sliced onion until it is very soft and dark brown, about 20 minutes.

When the beef finishes cooking, transfer it to a platter and allow it to cool before using your hands to shred it into very thin pieces.

Transfer the shredded beef to a large bowl, and add the garlic and lime juice.

Mix well, and then allow it to marinate for 30 minutes on the counter.

In a large skillet over high heat, heat 1 tablespoon of olive oil.

Working in batches, fry the shredded beef in a single layer until very browned and crispy, 4 to 7 minutes per batch.

Season with salt and pepper, and remove from the heat.

Repeat with the rest of the beef, adding more oil as necessary.

Serve each plate of vaca frita topped with some of the sautéed onions and a wedge of fresh lime.