Preheat the oven to 375 degrees. 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 diced onion and cook until softened. Stir in the minced garlic and cook for one minute more.

Add the Italian sausage to the skillet. Cook and break it up until browned and no longer pink. Season with salt and black pepper.

Pour the marinara sauce into the skillet and stir to combine with the sausage mixture. Simmer for a few minutes.

In a large bowl, mix the cooked ziti with the ricotta cheese and half of the mozzarella. Add the sausage sauce and stir until everything is evenly coated.

Transfer the mixture to a greased baking dish. Sprinkle the remaining mozzarella and the Parmesan cheese over the top.

Bake for 20 to 25 minutes until the cheese is melted and bubbly. Let it rest for five minutes, then garnish with fresh basil before serving.