Preheat the oven to 375°F (190°C).
Cook the ziti pasta according to package instructions until al dente. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add chopped onion and cook until softened, about 3-4 minutes.
Add minced garlic to the skillet and cook for 1 minute until fragrant.
In a large mixing bowl, combine cooked pasta, shredded chicken, marinara sauce, cooked onions and garlic, ricotta cheese, dried Italian seasoning, salt, and black pepper. Mix well to combine.
Transfer half of the pasta mixture into a 9×13 inch baking dish.
Sprinkle 1 cup of shredded mozzarella cheese evenly over the pasta layer.
Add the remaining pasta mixture on top and spread evenly.
Top the casserole with the remaining 1 cup of shredded mozzarella cheese and grated Parmesan cheese.
Bake uncovered in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly and the casserole is heated through.
Remove from the oven and let it rest for 5 minutes before serving.
Garnish with chopped fresh parsley and serve warm.