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.