Preheat the oven. Preheat the oven to 375°F (190°C). Lightly grease a 9 × 13-inch baking dish with olive oil or cooking spray and set it aside.
Cook the pasta. Bring a large pot of salted water to a boil. Add the ziti and cook until just shy of al dente, following the package instructions and reducing the cooking time by about 2 minutes. The pasta will continue cooking in the oven. Drain well and set aside.
Prepare the tomato sauce. Heat the olive oil in a large skillet over medium heat. Add the diced onion and cook for 4 to 5 minutes, until softened. Stir in the garlic and cook for another 30 seconds. Add the marinara sauce, crushed tomatoes, Italian seasoning, oregano, salt, and black pepper. Simmer for 8 to 10 minutes, stirring occasionally, to allow the flavors to combine.
Make the ricotta mixture. In a mixing bowl, combine the ricotta cheese, egg, half of the Parmesan cheese, 1 cup of shredded mozzarella, and chopped parsley. Stir until the ingredients are evenly combined and the mixture is creamy.
Combine the pasta and sauce. Add the drained ziti to the tomato sauce and toss until the pasta is thoroughly coated. Remove the skillet from the heat and allow it to cool slightly before assembling the casserole.
Assemble the baked ziti. Spread half of the sauced pasta in the prepared baking dish. Spoon the ricotta mixture evenly over the pasta, spreading it gently without needing to cover every piece. Add the remaining pasta on top. Sprinkle with the remaining 2 cups of mozzarella and the remaining Parmesan cheese.
Bake until bubbly. Cover the baking dish loosely with aluminum foil and bake for 20 minutes. Remove the foil and bake for another 10 to 15 minutes, until the cheese is melted, golden in spots, and bubbling around the edges.
Rest and serve. Remove the baked ziti from the oven and let it rest for 5 to 10 minutes before serving. Garnish with additional chopped parsley and serve warm while the cheese is still melted.