Heat the oven to 375°F, grease a 9-by-13-inch baking dish and boil the shells until flexible but still firm.
Mix the cooked chicken, ricotta, egg, 1/2 cup mozzarella, Parmesan, lemon zest, basil, salt and black pepper.
Spread 1 cup marinara in the dish and fill each drained shell with about 2 tablespoons chicken mixture.
Arrange the shells in the dish, spoon over the remaining marinara and sprinkle with the remaining mozzarella.
Cover with foil and bake for 25 minutes, then uncover and bake until bubbling, about 10 minutes.
Rest the shells for 5 minutes before serving.