Cut beef into 2″ cubes, lightly salt and pepper and, working in batches to maintain a consistent temperature, brown all sides over medium-high heat in an oiled, preheated skillet. Transfer beef to a slow cooker and combine with salsa. Cover and cook over low heat for four hours. Shred beef using two forks, cover and let stand for 10 minutes.
Remove beef from liquid using slotted spoon or tongs before serving.
Serve inside warm tortillas topped cheese, onion, cilantro and sprinkled with the juice of freshly squeezed limes.