Preheat the Oven:

Preheat the oven to 375°F (190°C). Lightly grease a baking dish with cooking spray.

Prepare the Filling:

Heat olive oil in a skillet over medium heat. Sauté diced onion and minced garlic until fragrant.

Add ground beef and Italian sausage. Cook until browned and crumbled. Drain excess fat if necessary.

Combine Ingredients:

In a mixing bowl, combine the cooked meat mixture with rice, diced tomatoes, breadcrumbs, Parmesan cheese, half the mozzarella, Italian seasoning, salt, pepper, and red pepper flakes (if using). Mix well.

Stuff the Peppers:

Fill each bell pepper with the prepared mixture, packing it firmly. Place the peppers upright in the baking dish.

Bake:

Spoon some tomato sauce or diced tomatoes over the tops of the peppers. Cover the dish with aluminum foil and bake for 45 minutes.

Add Cheese and Finish Baking:

Remove foil, sprinkle the remaining mozzarella over the peppers, and bake for an additional 10 minutes, or until the cheese is melted and bubbly.

Garnish and Serve:

Let the peppers cool slightly, then garnish with fresh basil and parsley. Serve warm.