Heat the oven to 375°F (190°C) and lightly coat a baking dish with olive oil or cooking spray.

Cut the peppers in half lengthwise or slice off their tops, then remove the seeds and membranes. For softer peppers, parboil them in boiling water for 3–5 minutes and drain well; otherwise, use them raw.

Warm the olive oil in a skillet over medium heat. Cook the onion for 4–5 minutes until softened, then add the garlic and cook for about 30 seconds.

Add the ground beef and cook until browned, breaking it into small pieces as it cooks. Drain excess fat if necessary.

Stir in the cooked rice, drained diced tomatoes, 1/2 cup tomato sauce, Italian seasoning, oregano, salt, pepper, and Parmesan. Simmer for 2–3 minutes, taste, and adjust the seasoning.

Spread the remaining 1/2 cup tomato sauce over the bottom of the prepared baking dish. Arrange the peppers cut-side up and spoon the meat-and-rice filling into each one.

Cover the dish tightly with foil and bake for 30–35 minutes, until the peppers are tender. Keeping the dish covered helps raw peppers steam in the sauce.

Remove the foil and sprinkle the mozzarella evenly over the peppers. Bake for another 8–12 minutes, until the cheese is melted and lightly browned.

Let the stuffed peppers rest for 5 minutes before serving. Finish with chopped parsley or basil for a fresh, colorful garnish.