Season beef with salt and pepper, to taste. Using a medium skillet, brown beef and onion over medium heat until no longer pink. While beef is browning, bring 1 cup of water to a boil in a small saucepan and add the rice. Give it a good stir, cover and cook per package directions (usually about 5 minutes).
Drain meat on a paper towel-lined plate to remove any excess grease. Wipe out skillet and place beef back in skillet. Add the rice, tomatoes, black beans, 3/4 cup water, taco seasoning and cilantro. Mix until meat is well combined. Add cheese and mix well.
Slice peppers in half and discard stems, seeds and membrane carefully. Spoon beef mixture into the peppers and bake for about 20 minutes. Garnish with green peppers and extra cilantro.