Heat the vegetable oil in a large skillet over medium-high heat. Season the beef with the cumin, 1 teaspoon salt and 1/4 teaspoon pepper. Add to the skillet and cook until golden brown, about 3 minutes per side.
Transfer the meat to a 6-quart slow cooker, then add the salsa, onion and garlic. Cover and cook on high, 6 hours.
Remove the meat to a cutting board and shred with 2 forks, discarding the excess fat. Return the meat to the slow cooker and stir in the cilantro.
Serve the meat in tortillas and top as desired. Serve with lime wedges.