Preheat to 350 degrees.
Trim ends of zucchini and cut in half lengthwise. Scoop out centers leaving 1/4” thick shells, reserving pulp.
Chop pulp and set aside.
Brown ground beef and drain.
Stir in pulp, 1/2 c. marinara sauce, bread crumbs, Parmesan, oregano, salt, pepper and egg. Mix well.
Place zucchini shells in baking dish and spoon in beef mixture. Pour remaining marinara sauce over top and cover with foil.
Bake 30-35 minutes or until tender. Remove foil, top with mozzarella cheese and bake 5 minutes more until cheese melts.