1. Fill a deep skillet with water and bring it to a boil. Prick the sausages with a fork, add them to the pan, and simmer for 2 minutes, or until the casings turn pale. Drain the sausages, and let cool. Remove the casings and break up the meat.

2. Drain the skillet and return to the heat. Add the sausage and oil and cook for 5 minutes. Add the onions and garlic and cook for another 3 minutes, or until softened.

3. Stir in the tomatoes with their juices and the basil. Season with salt and pepper. Allow the sauce to simmer for 10 minutes, or until it has thickened, stirring occasionally.

4. Meanwhile, preheat the oven to 350°F (180°C). Bring a large pot of salted water to a boil, and add the ziti. Cook until al dente, about 3 minutes before the time recommended on the package. Drain and return the pasta to the pot.

5. Mix most of the sausage and tomato sauce with the ziti. Spread in the casserole(s), then spoon the reserved sauce over the top. Sprinkle with the mozzarella.

6. Bake for about 20 minutes, or until the sauce bubbles, and the cheese is melted and golden.