Heat the oven to 180C/350F/gas mark 4. Heat the oil in a large frying pan, add the aubergine and cook for eight minutes on medium to high heat until golden brown, stirring occasionally. Remove with a slotted spoon and drain on kitchen paper. Add the carrot and celery, fry for eight minutes, and transfer to kitchen paper. Lower the heat to medium, add the onion and garlic and cook, stirring, for five minutes. Stir in the orzo and tomato paste, and cook for two minutes.
Remove the pan from the heat and pour in the stock, fresh oregano and lemon zest. Add the cooked veg, mozzarella, parmesan, a teaspoon of salt and half a teaspoon of pepper. Mix well and transfer to a 21cm x 27cm ovenproof dish. Arrange the tomatoes on top, sprinkle with dried oregano and season. Bake for 40 minutes, until all the liquid has been absorbed and the pasta is cooked. Remove, leave to settle for five minutes and serve hot.