This sunny gratin was born one hot summer near Vienna, Austria, when I wanted to make a meatless dish with what I had on hand. Juicy tomatoes, crunchy peppers, melting mozzarella, fragrant olives and my favourite oregano: the result was so successful that it has become my summer dish par excellence. Simple, generous and full of sunshine, it can be prepared in advance and is easy to keep until it's time to put it in the oven. Enjoy it as a vegetarian dish or as the perfect accompaniment to a barbecue.