Bring a large pot of lightly salted water to the boil. Add penne and cook for 10 to 12 minutes or until al dente. Drain. Lightly grease a medium baking dish.
Preheat oven to 180 C / Gas 4. In a liquidiser or food processor, blitz together the Parmesan cheese and crackers. Set aside.
Heat the oil in a frying pan over medium heat. Add the onion and stir until tender. Mix in the courgettes, tomato and garlic. Season with oregano, basil, celery flakes, salt and pepper. Continue to cook and stir until the courgettes are tender.
Mix the penne with the vegetables and mozzarella in the prepared dish. Sprinkle over the Parmesan cheese evenly.
Bake 25 minutes in preheated oven or until the top is lightly browned. Allow to sit 5 minutes before serving.
Parmesan cheese is not truly vegetarian, as it contains animal rennet. To make this dish 100% vegetarian, omit the cheese or find a suitable vegetarian substitute made without animal rennet. In supermarkets look for the 'parmesan style hard cheeses' which are suitable for vegetarians.