Preheat your oven to 375℉ (190℃).
In a large pot of boiling salted water, cook the ziti until al dente, according to package instructions. Drain well.
In a large skillet, heat the olive oil over medium heat. Add the onion and garlic, and cook until the onion is translucent, about 5 minutes. Add the Italian sausage and cook until browned. Stir in the marinara sauce, dried basil, dried oregano, and season with salt and pepper to taste. Simmer for about 10 minutes.
In a bowl, combine the ricotta cheese, egg, 1 cup of the mozzarella cheese, and the Parmesan cheese.
In a large baking dish, spread a layer of the meat sauce. Layer half of the cooked ziti, followed by all of the cheese mixture, then half of the remaining sauce. Add the remaining ziti, then the remaining sauce. Top with the remaining mozzarella cheese.
Cover the dish with foil and bake for 20 minutes. Then, remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.
Allow the ziti to cool for a few minutes before serving. Garnish with fresh basil or parsley, and enjoy!