In a large saucepan, combine the flank steak with the bell pepper, halved onion and bay leaf. Add enough water to cover and bring to a boil. Simmer over moderate heat for 20 minutes. Transfer the flank steak to a work surface and let the steak cool. Strain the broth and reserve for another use. Shred the meat and transfer it to a bowl.
Using the side of a large knife, mash the garlic to a paste with 1/2 teaspoon of salt. Stir the paste into the meat, along with the lime juice, olive oil and sliced onion. Let stand at room temperature for at least 30 minutes or up to 1 1/2 hours.
Heat a large, flat griddle until very hot. Working in batches, spread the shredded beef on the griddle in a thin layer and season with salt and pepper. Cook over high heat, turning once or twice, until sizzling and crispy in spots, about 6 minutes per batch. Transfer to a platter and serve.