Preheat the oven to 350°F. Fill a 9x13-inch baking dish with about 1/2 inch of water.

Trim 1/4 inch from the top of each bell pepper, then remove the stem, ribs and seeds. Place the peppers cut-side down in the baking dish, cover the dish with foil, and bake for 30 minutes, then set aside to cool. Drain the water from the pan.

Heat the oil in a large skillet over medium heat. Add the onions to the pan and cook until translucent, stirring occasionally, approximately 3 minutes. Add the sausage and cook until browned, breaking the meat into small pieces while stirring, about 5 minutes. Stir in the garlic and cook for an additional 30-60 seconds, until fragrant. Remove from the heat and drain off any excess fat.

Stir in the rice, tomatoes, half the tomato sauce, Italian seasoning, parsley, salt and pepper.

Lightly grease the baking dish with olive oil. Place the peppers cut-side up in the pan, then divide the sausage mixture evenly between the peppers. Top each pepper with a spoonful of the remaining tomato sauce, spreading it evenly. Cover with foil and bake for 25 minutes, then remove the foil, top each pepper with an even sprinkle of mozzarella cheese, and bake for an additional 15-20 minutes, or until peppers are tender.

Sprinkle with additional parsley and serve.