Cook the pasta in boiling water for 10–12 minutes, or according to the packet instructions, until al dente. Drain.
Meanwhile, grill the sausages until browned, turning once. Cool slightly, then cut into chunky slices and set aside.
Heat the oil in a saucepan over a moderate heat and fry the onion for 3 minutes until starting to colour. Add the garlic and sliced sausages and cook for a few minutes. Increase the heat, add the eggplant and zucchini, and cook, stirring, for 5 minutes, or until the eggplant begins to soften.
Add the tomato passata, then stir in the oregano and seasoning to taste.
Cover and simmer, stirring occasionally, for 15 minutes, or until the eggplant is tender. Add the olives and the cherry tomatoes and stir well. Cover again and cook for a further 5 minutes.
Finally, mix in the pasta until it is thoroughly coated. Stir in the chopped parsley. Serve immediately, sprinkled with oregano leaves and parmesan, if you like.