Preheat oven to 375°F (190°C). Lightly grease a baking dish.

In a skillet over medium heat, brown the ground beef until cooked through. Drain excess fat and add the chopped onion; cook until soft.

Stir in rice, diced tomatoes, tomato sauce, garlic powder, salt, and pepper. Simmer for about 10 minutes.

Slice bell peppers in halves or quarters and remove seeds. Layer half of the meat mixture in the baking dish, place pepper pieces on top, then spoon the remaining mixture over them.

Cover with foil and bake for 30 minutes. Remove foil, sprinkle cheese on top, and bake uncovered for another 10 minutes until cheese is melted.