Preheat the oven to 375°F (190°C). Cut the tops off the bell peppers and remove the seeds.

In a skillet, cook the ground beef and onion over medium heat until browned. Drain excess fat.

Stir in the cooked rice, diced tomatoes, garlic powder, chili powder, salt, and pepper. Mix well and heat through.

Fill each bell pepper with the meat and rice mixture, packing it down gently. Top with shredded cheese.

Place the stuffed peppers in a baking dish and cover with foil. Bake for 25 minutes, then remove the foil and bake for an additional 5 minutes until the cheese is bubbly.