Bring a large pot of salted water to a boil and cook the ziti until just al dente. Drain and set aside.

Heat the olive oil in a large skillet over medium heat. Add the onion and cook until softened, about 3 minutes. Stir in the garlic and cook for 1 minute more.

Add the Italian sausage to the skillet. Break it up with a spoon and cook until browned and cooked through. Drain any excess fat if needed.

Pour the marinara sauce into the skillet with the sausage. Season with salt and black pepper, then simmer for 5 minutes. Remove from heat.

Preheat the oven to 375°F. Spread a thin layer of the sausage sauce in the bottom of a 9x13 inch baking dish.

Add half the cooked ziti, then dollop half the ricotta over the pasta. Sprinkle with half the mozzarella and Parmesan. Repeat the layers with the remaining pasta, sauce, ricotta, and cheeses.

Bake uncovered for 20 to 25 minutes until the cheese is melted and lightly browned on top. Let rest for 5 minutes before serving.