Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds. Rinse and drain them.
Heat olive oil in a large skillet over medium heat, then add onion and garlic. Sauté until the onion is translucent.
Add ground beef to the skillet and cook until browned thoroughly.
Stir in cooked rice, tomato sauce, diced tomatoes, Italian seasoning, salt, and black pepper. Let it simmer for 5 minutes.
Stuff each bell pepper with the beef and rice mixture.
Place the stuffed peppers upright in a baking dish.
Pour about 1/4 inch of water into the bottom of the baking dish.
Cover the dish with foil and bake for 35 minutes.
Remove the foil, sprinkle mozzarella and Parmesan cheese on top of each pepper.
Bake uncovered for an additional 10-15 minutes until the cheese is melted and bubbly.
Let the stuffed peppers cool slightly, garnish with fresh parsley, and serve.