Preheat the Oven: Heat the oven to 375°F (190°C).
Prepare the Peppers: Slice off the tops of the bell peppers and carefully discard the seeds and inner membranes. Lightly coat the inside of each pepper with olive oil, making sure to reach all sides. Season them with a pinch of salt for added flavor. Arrange the peppers upright in a baking dish and pre-bake them in the oven for about 10 minutes. This step helps soften the peppers slightly, making them easier to fill and ensuring they bake evenly later.
Cook the Meat: Heat a tablespoon of olive oil in a pan over medium heat.. Cook the ground beef, Italian sausage, onion, and garlic until browned. Drain excess fat.
Make the Filling: In a large bowl, mix the cooked meat, rice, 1 cup marinara sauce, Italian seasoning, half of the mozzarella, and half of the Parmesan. Season with salt and pepper.
Stuff the Peppers: Spoon the filling evenly into the pre-baked peppers. Press gently to pack.
Add Sauce and Cheese: Spoon extra marinara sauce over the stuffed peppers. Top with remaining mozzarella and Parmesan cheese.
Bake: Place a sheet of foil over the baking dish and bake for 25 minutes. Then, uncover and continue baking for an additional 10 minutes, or until the cheese becomes bubbly and golden.
Serve: Garnish with fresh basil or parsley if desired. Serve hot.