Bring a pan of water to the boil. Lower the tomatoes into the boiling water and boil for ten seconds, then remove them from the pan using a slotted spoon and immediately plunge them into a bowl of ice-cold water.
When the tomatoes have cooled, peel and discard the skins. Chop the tomatoes and discard the seeds. (Alternatively freeze the seeds and use them in stocks and sauces.) Continue to chop the tomato flesh to a pulp.
Heat half of the olive oil in a saucepan over a medium heat. Add the shallots, thyme leaves and garlic and fry for 2-3 minutes, or until softened but not coloured.
Add the remaining olive oil and the chopped tomato pulp, then stir well to combine.
Cut a disc of greaseproof paper 1cm/½in smaller than the diameter of your saucepan. Cover the surface of the tomato sauce with the greaseproof paper disc (this will prevent the sauce from drying out as it reduces and cooks). Reduce the heat to its lowest setting and cook gently for at least an hour, or until the sauce has thickened.
When the mixture has cooked and thickened, set aside to cool slightly. (NB: The sauce can be split into portions and frozen until needed.)