Quarter the tomatoes and put them in the bowl of a food processor. Process until soupy.
Pour through a fine sieve into a pot using a rubber spatula. Push through as much of the tomato mixture as possible.
Over a high heat bring to a boil, then turn down to a strong simmer. Allow water to boil off, this will leave you with a thick paste.
Preheat oven to 400ºF (200ºC). Once the oven is heated turn down to 200ºF (95ºC).
Pour the paste onto a clean baking tray.
Place the tray in the oven and allow the paste to dry out and thicken further.
Keep an eye on this as timing will differ according to the consistency of your paste.
Remove and cool once your preferred consistency is reached.