- In a large pot of salted boiling water, cook the pasta until al dente. Add the onion 5 minutes before the end of cooking the pasta. Drain and oil lightly.
- With the rack in the middle position, preheat the oven to 190 °C (375 °F).
- Return the pasta to the pot. Add the tomatoes, sausages, Parmesan cheese, green onions, and parsley. Toss to combine. Season with salt and pepper.
- Spoon into individual ramekins or in one large baking dish and top with the mozzarella. Bake for about 20 minutes. Finish cooking under the broiler, if desired.