Season the beef chuck roast with salt, pepper, cumin, and chili powder.
In a large skillet, heat oil over medium heat and brown the beef on all sides.
Remove the beef and set it aside.
In the same skillet, sauté onions and garlic until soft.
Add the beef broth to the skillet, scraping up any browned bits.
Return the beef to the skillet and cover.
Cook on low heat for 3-4 hours, or until the beef is fork-tender.
Remove the beef from the skillet and shred it using two forks.
Return the shredded beef to the skillet to soak up the juices.
Warm the corn tortillas and assemble the tacos with shredded beef and desired toppings.