Preheat the oven to 375°F (190°C).

Cut the tops off the bell peppers and discard the stems and seeds. Rinse them under cool water and set aside.

In a large skillet, heat the olive oil over medium heat. Add the chopped onion and minced garlic, and sauté for about 3-4 minutes until the onion turns translucent.

Add the ground beef to the skillet and cook for 5-7 minutes until fully browned, breaking it into bite-sized pieces.

Reduce heat and mix in cooked rice, tomato sauce, diced tomatoes, Italian seasoning, salt, and black pepper. Simmer for 5 minutes.

Stuff each pepper with the filling mixture, being careful not to overfill. Place each stuffed pepper upright in a baking dish.

Pour about 1/4 inch of water into the bottom of the baking dish. Cover with aluminum foil and bake for 35 minutes.

Remove the foil and sprinkle mozzarella and Parmesan cheese on top. Bake uncovered for an additional 10-15 minutes until cheese is melted.

Once out of the oven, let the peppers cool briefly, then garnish with fresh parsley before serving.