In a small bowl, combine ground cumin, chili powder, smoked paprika, dried oregano, salt, black pepper, garlic powder, onion powder, and cayenne pepper to create the spice blend.
Rub the spice blend 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 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 evenly over the beef.
Pour the orange juice, lime juice, and beef broth over the beef and onions in the slow cooker.
Add the bay leaves 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 leaves from the slow cooker.
Using two forks, shred the beef directly in the slow cooker, mixing it with the cooking juices and onions.
Optional: For crispy edges, spread the shredded beef on a baking sheet and broil in the oven for 3-5 minutes, turning once, until edges are crisp.
Serve the beef carnitas warm in tacos, burritos, or bowls with your favorite toppings.