Cut the tops off the bell peppers and remove the seeds and membranes so they can stand upright.
In a large skillet over medium heat, cook the ground beef with the diced onion and minced garlic until the beef is browned. Drain any excess fat.
In a mixing bowl, combine the cooked beef mixture with the rice, black beans, salsa, chili powder, cumin, and salt. Stir until everything is evenly mixed.
Spoon the filling into the prepared peppers, packing it gently but firmly.
Place the stuffed peppers upright in the slow cooker. Cover and cook on low for 5 hours.
Sprinkle the cheese over the tops of the peppers, cover again, and cook for 15 more minutes until the cheese is melted.
Remove the peppers from the slow cooker and top with chopped cilantro before serving.