Preheat the grill to medium.
In a small bowl mix together well all the ingredients for the gratin topping and set aside.
In a saucepan cook the Penne in salted boiling water for 6 minutes, drain and set aside.
Meanwhile, in a large frying pan heat the oil before adding the garlic, onions, courgette/zucchini and mushrooms. Sauté the vegetables for 4–5 minutes until soft.
Mix in the pasta, pasta sauce, cheese and water, bring to the boil, mixing well to combine.
Transfer the pasta and sauce into a small ovenproof dish, sprinkle with the gratin topping.
Place under the grill on the middle shelf until golden brown.