1. Preheat the oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish and set aside.

2. Bring a large pot of salted water to a boil. Add the ziti and cook until just shy of al dente, about 1–2 minutes less than the package directions. Drain and set aside.

3. While the pasta cooks, heat the olive oil in a large skillet over medium-high heat. Add the Italian sausage, breaking it into crumbles, and cook until browned and cooked through.

4. Stir in the chopped onion and cook until softened, 3–4 minutes. Add the minced garlic and cook for 30–60 seconds, just until fragrant.

5. Pour in the marinara sauce and tomato sauce (or crushed tomatoes). Stir in the Italian seasoning, crushed red pepper flakes (if using), salt, and black pepper. Simmer the sauce for 5–10 minutes to thicken slightly and blend the flavors.

6. In a medium bowl, combine the ricotta cheese, 1/4 cup of the grated Parmesan, and 1/2 cup of the shredded mozzarella. Season with a small pinch of salt and pepper and stir until smooth.

7. Add the cooked ziti to the skillet with the sausage sauce (or combine in a large bowl if needed), reserving about 1 cup of sauce. Toss gently until all the pasta is evenly coated.

8. Spread a thin layer of the reserved sauce over the bottom of the prepared baking dish. Add half of the sauced pasta in an even layer. Dollop half of the ricotta mixture over the pasta, then sprinkle with about 1/2 of the remaining mozzarella and a little Parmesan.

9. Repeat with the remaining pasta, ricotta mixture, and cheeses, finishing with the last of the reserved sauce and the remaining mozzarella and Parmesan on top.

10. Cover the baking dish tightly with foil and bake for 20 minutes. Remove the foil and bake for another 10–15 minutes, or until the cheese is melted, bubbly, and lightly golden.

11. Let the baked ziti rest for about 10 minutes before serving. Sprinkle with chopped fresh parsley or basil, slice into squares, and serve warm.