Sweat the carrot and the onion gently in the oil in a covered saucepan. Add the tomatoes and tomato purée, the stick of celery, the parsley stalks and the stock.
Let it simmer over a low heat for about an hour, skimming from time to time. Add cold water if necessary - there should be 500 ml (scant pint) liquid left after reducing. Ten minutes before the end of