Preheat oven to 375°F. In a skillet over medium heat with a drizzle of olive oil, cook the sausage and onion until browned, about 8 minutes. Add garlic and Italian seasoning, cook 1 minute. Stir in ½ cup marinara, remove from heat.

In a bowl, combine the sausage mixture, rice, egg, Parmesan, salt, and pepper. Mix well.

Cut tops off peppers and remove seeds. Spoon filling into each pepper, packing gently. Stand peppers in a baking dish, drizzle with olive oil, and pour remaining ½ cup marinara over and around them.

Cover with foil and bake 30 minutes. Uncover, top each pepper with mozzarella, and bake 10-15 minutes more until cheese is melted and bubbly. Let rest 5 minutes before serving.