In a small bowl, combine ground cumin, dried oregano, smoked paprika, chili powder, garlic powder, onion powder, salt, and black pepper.
Rub the spice mixture all over the beef chuck roast pieces to coat evenly.
Heat olive oil in a large skillet over medium-high heat. Sear the beef pieces on all sides until browned, about 2-3 minutes per side. Transfer the beef to the slow cooker.
Add the sliced onion and minced garlic to the slow cooker, spreading them over and around the beef.
Pour the orange juice, lime juice, and beef broth over the beef and onions in the slow cooker.
Add the bay leaf to the slow cooker.
Cover and cook on low for 8 hours, or until the beef is very tender and easily shredded with a fork.
Remove the bay leaf and transfer the beef to a large bowl. Shred the beef using two forks.
Return the shredded beef to the slow cooker and stir to combine with the cooking juices.
Serve the beef carnitas warm in tacos, burritos, or bowls with your favorite toppings.