Coat 6-quart slow cooker with cooking spray.
In a large pan, heat olive oil over medium-high heat. Season the beef with cumin, salt and pepper. Add to the pan and cook for about 3 minutes on each side, until browned.
Place meat in the slow cooker. Add onions, garlic, and salsa verde. Cover and cook over low heat for 8 hours.
Shred the meat with 2 forks.
Use tongs to place some of the meat in each tortilla. Top with queso fresco, cilantro, and lime juice.