1In small bowl, beat lime juice, oil, vinegar, cumin, paprika, chili powder, red pepper, salt and pepper.
2In 4- to 6-quart oval slow cooker, arrange onion chunks. Place beef over onion. Pour lime juice mixture over beef. Using pastry brush, brush beef with any remaining mixture from bowl. Sprinkle garlic over top.
3Cover; cook on Low heat setting 8 to 9 hours.
4Remove beef from slow cooker; place on large plate. Using 2 forks, shred beef. Return beef to slow cooker; stir well to mix beef with sauce. Serve immediately.