Combine the beef and bay leaf in a pot, cover with salted water, and cook over low heat until the beef is tender, 1 to 1 1/2 hours.
Remove the beef from the liquid and cool to room temperature.
Cut the beef into 2-inch (5 cm) pieces and shred with your fingers or a pair of forks.
Combine the shredded beef with the lime juice, lemon juice, garlic, salt, and pepper in a non-reactive bowl and marinate covered in the refrigerator for at least 1 hour or overnight.
Heat the oil in a large heavy skillet over moderate heat until it is very hot.
Drain the excess liquid from the beef and fry in the oil, stirring frequently, for 6 to 8 minutes.
Add the onion and cook, stirring frequently, until the beef is crisp.