1. Cut zucchini in half and a thin slice from the bottom to allow zucchini to sit flat. Scoop out pulp, leaving 1cm shells.
2. Steam or microwave the zucchini shells on high for 3 minutes or until crisp-tender; drain and set aside.
3. Panfry the beef and onion over medium heat until meat has browned; drain. Stir in the marinara sauce, egg, bread crumbs, salt, pepper and 1/2 cup cheese.
4. Fill each shell with stuffing. Place under a grill until the cheese starts to bubble and turn golden. Serve with additional marinara sauce.