Cook the pasta: Bring a large pot of salted water to a boil. Cook the ziti according to package directions until al dente. Drain, rinse with cold water, and set aside.

Make the sauce: In a large skillet, heat olive oil over medium heat. Sauté the chopped onion and garlic for about 3 minutes until softened. Stir in the crushed tomatoes, Italian seasoning, chopped basil, salt, and pepper. Simmer on low heat for 20 minutes, stirring occasionally. Remove from heat and let it cool slightly. (You should have around 2½ cups of sauce.)

Preheat the oven: Preheat to 375°F (190°C). Lightly grease a 9×13-inch (23×33 cm) baking dish.

Mix the cheese filling: In a large mixing bowl, combine 1 cup ricotta, ½ cup mozzarella, ¼ cup Parmesan, and the egg. Season with salt and pepper. Stir in 1½ cups of the prepared tomato sauce.

Combine with pasta: Add the cooked pasta to the cheese mixture and toss until well coated.

Assemble the casserole: Spread half of the pasta mixture into the prepared baking dish. Dot with ½ cup of the remaining ricotta. Layer the remaining pasta and sauce over the top, pressing down slightly. Finish with spoonfuls of the remaining ricotta and sprinkle with the remaining mozzarella and Parmesan.

Bake: Bake uncovered for 20–25 minutes, or until the cheese is golden and bubbling around the edges.

Serve: Let stand for 5 minutes before serving. Garnish with fresh basil if desired.