Preheat the oven to 375°F (190°C). Cut the bell peppers in half lengthwise and remove the seeds.

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

Stir in the cooked rice, diced tomatoes, garlic powder, Italian seasoning, salt, and pepper. Mix well and remove from heat.

Spoon the filling mixture into each bell pepper half. 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.