In a small bowl, combine ground cumin, dried oregano, smoked paprika, chili powder, garlic powder, onion powder, salt, and black pepper to create the spice mix.
Rub the spice mix evenly over all sides of the beef chuck roast chunks.
Heat olive oil in a large skillet over medium-high heat. Sear the beef chunks for 2-3 minutes on each side until browned. Transfer the beef to the slow cooker.
Add the sliced onion and minced garlic on top of the beef in the slow cooker.
Pour beef broth, fresh orange juice, and fresh lime juice over the beef and onions. Add the bay leaf.
Cover and cook on low for 8 hours, or until the beef is very tender and easily shredded with a fork.
Remove the beef chunks from the slow cooker and shred them using two forks. Discard the bay leaf.
Return the shredded beef to the slow cooker and stir to combine with the cooking juices.
Optional: For crispy edges, spread the shredded beef on a baking sheet and broil in the oven for 5-7 minutes, stirring once halfway through.
Serve the beef carnitas warm with your favorite tortillas, toppings, and sides.