Place the whole tomatoes (with stalks attached) onto a baking tray with 3/4 of the oil and cook in a preheated oven set to 180°C/gas mark 4 for an hour
Heat the remaining oil in a heavy based saucepan and gently fry the diced onions in the pan for 10 minutes without colouring
Add the chopped herbs and tomato purée and cook for a further 5 minutes, stirring occasionally