Start by trimming any excess fat from the 4-pound chuck roast and cut it into large chunks.
In a small bowl, combine garlic powder, chili powder, cumin, Mexican oregano, and kosher salt. Rub this spice mixture all over the beef chunks.
In a large slow cooker or Dutch oven, heat avocado oil over medium heat. Add the diced onion and sauté until translucent, about 5 minutes.
Optionally, brown the seasoned beef chunks in the pot with the onions for about 5 minutes on each side.
Pour in the beef broth and stir in tomato paste, then add diced tomatoes or green chilis. Mix well.
Cover and cook on low for 8 hours or high for 4 hours, until the beef is fork-tender.
Remove the beef from the pot and shred it using two forks. Return the shredded beef to the pot and stir it into the sauce.
Serve in tacos, burritos, or over rice, garnished with cilantro, avocado, or lime.