Cut zucchini in half lengthwise; cut a thin slice from the bottom of each with a sharp knife to allow zucchini to sit flat. Scoop out pulp, leaving 1/4-in. shells. , Place shells in an ungreased 3-qt. microwave-safe dish. Cover and microwave on high for 3 minutes or until crisp-tender; drain and set aside. , Meanwhile, in a large skillet, cook beef, onion and zucchini pulp over medium heat until meat is no longer pink; drain. Remove from the heat; stir in the marinara sauce, egg, bread crumbs, salt, pepper and 1/2 cup cheese., Spoon about 1/4 cup into each shell. Microwave, uncovered, on high for 4 minutes. Sprinkle with remaining cheese. Microwave 3-4 minutes longer or until a thermometer inserted into filling reads 160° and zucchini are tender. If desired, broil zucchini 3 in. from the heat for 3-5 minutes or until the cheese is golden brown. Serve with additional marinara sauce.