Heat the vegetable oil in a large skillet over medium-high heat. Rub the beef with cumin, 1 teaspoon of salt and 1/4 teaspoon of pepper. Add the beef to the skillet and brown each side for 2-3 minutes.
Transfer the meat to a 6-quart slow cooker and add the salsa verde, onion, and garlic on top. Cover and cook on low for 6-8 hours.
Remove the meat from the slow cooker and shred it with 2 forks. Return it to the slow cooker and stir in the cilantro. Using a slotted spoon remove the meat and use for tacos.
Serve the meat in warm tortillas and top with preferred toppings. Serve with lime wedges.