In a liquid measure, combine spaghetti sauce, Italian seasoning, and instant onion. Mix well and pour one cup into a 11"x7" dish; set aside. In a mixing bowl, combine egg, cottage cheese, tuna, mozzarella cheese, parmesan cheese, parsley, and salt. Mix well. Fill each jumbo shell evenly with tuna mixture. Arrange shells on sauce in dish. Pour remaining sauce over shells, covering shells completely. Microwave on 70% for two minutes; turn dish one-half turn. Microwave again for four minutes or until heated through.