Preheat the oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish.
Bring a large pot of lightly salted water to a boil. Add the ziti pasta and cook for 8–10 minutes, until tender but still firm to the bite. Drain and rinse the pasta.
In a large bowl, combine the cooked ziti, ricotta cheese, mozzarella cheese, 1½ cups of spaghetti sauce, and beaten egg. Mix until the ingredients are evenly combined.
Transfer the pasta mixture into the prepared baking dish and spread it into an even layer.
Pour the remaining spaghetti sauce over the top and sprinkle with Parmesan cheese.
Bake uncovered for 30 minutes, until the cheese is melted and the casserole is heated through.
Remove from the oven and let the baked ziti stand for 15 minutes before serving.