Preheat your oven to 375 degrees F. Cut the tops off the bell peppers and remove the seeds and membranes. Place them cut-side up in a baking dish.
Heat olive oil in a large skillet over medium-high heat. Add the diced onion and cook for 3 minutes until softened. Add the ground beef and break it apart, cooking for 6 to 7 minutes until fully browned. Drain any excess fat.
Stir in the minced garlic, Italian seasoning, salt, and black pepper. Cook for 30 seconds until fragrant.
Remove the skillet from heat. Stir in the cooked rice, drained diced tomatoes, and 1 cup of the shredded mozzarella cheese until everything is well combined.
Spoon the beef and rice filling into each bell pepper, packing it in firmly and mounding it at the top. Spoon marinara sauce over the top of each stuffed pepper.
Cover the baking dish tightly with aluminum foil. Bake for 35 minutes until the peppers are tender.
Remove the foil and sprinkle the remaining 1/2 cup mozzarella cheese on top of each pepper. Bake uncovered for another 10 minutes until the cheese is bubbly and golden. Let cool for 5 minutes before serving.