Preheat the oven to 400°F. Cook the rice according to the package directions.
Slice about 1/4 inch off the top of each pepper, then remove the stems, ribs and seeds. Pour about 1/2 inch of water into a baking dish just large enough to hold the 6 peppers. Stand the peppers upside down in the water, cover the dish tightly with foil and bake for 20 minutes.
While the peppers bake, heat the olive oil in a large nonstick skillet over medium-high heat. Add the onion and cook for 3 to 4 minutes. Push the onion to one side of the pan. Add the ground beef in chunks, season it with 1 teaspoon of salt and 1/4 teaspoon of pepper, and let it brown on the bottom for about 3 minutes.
Break up the beef, stir it into the onion and cook for 2 minutes. Add the garlic and cook for about 1 minute more, until the beef is cooked through. Take the pan off the heat and drain off the excess fat.
Stir in the drained diced tomatoes, 1/2 cup of the tomato sauce, the cooked rice, 3 tablespoons of parsley and the Italian seasoning. Taste and add more salt and pepper if needed.
Reduce the oven temperature to 350°F. Turn the peppers upright in the dish and sprinkle a pinch of salt inside each one. Fill the peppers with the beef and rice mixture and pour the remaining tomato sauce over the tops.
Cover the dish with foil and bake for 20 minutes.
Uncover, sprinkle the mozzarella over the peppers and bake uncovered for 10 to 20 minutes more, until the cheese is melted and the peppers are as tender as you like. Garnish with more parsley and serve warm.