Place the beef chuck roast in the slow cooker. Add the sliced onion, minced garlic, salsa, beef broth, chili powder, cumin, and oregano. Season with salt and black pepper.
Cover and cook on low for 8 hours until the beef is very tender and pulls apart easily.
Use two forks to shred the beef directly in the slow cooker. Stir it into the juices and let it sit for a few minutes to absorb more flavor.
Warm the black beans in a small saucepan or microwave if you prefer them hot.
Divide the cooked rice among four bowls. Top each bowl with shredded beef, black beans, cheddar cheese, avocado slices, cilantro, a lime wedge, and sour cream.