Preheat your oven to 375°F (190°C).
Cut the tops off the bell peppers and remove the seeds and membranes. Set aside.
In a large skillet, cook the ground beef over medium heat until browned. Drain any excess fat.
Add the chopped onion to the skillet and sauté until translucent, about 3-4 minutes.
Stir in the cooked rice, diced tomatoes, black beans (if using), garlic powder, chili powder, cumin, salt, and pepper. Mix well and cook for another 5 minutes.
Remove the skillet from heat and stir in half of the shredded cheese.
Fill each bell pepper with the meat and rice mixture, packing it down gently. Place the stuffed peppers in a baking dish.
Cover the baking dish with aluminum foil and bake for 25 minutes.
Remove the foil, sprinkle the remaining cheese on top of the peppers, and bake for an additional 5-10 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let cool for a few minutes before serving. Garnish with fresh parsley or cilantro if desired.