Season the beef chuck with salt, pepper, and cumin.
Heat a skillet and brown the beef on all sides.
Transfer the beef to a slow cooker.
Add beef broth, diced onion, and minced garlic to the slow cooker.
Cook on low for 4 hours until tender.
Remove the beef and shred it with two forks.
Return shredded beef to the slow cooker and mix well.
Warm corn tortillas on a skillet.
Fill tortillas with shredded beef.
Top with chopped onions, cilantro, and a squeeze of lime juice.