Place all of the ingredients into a bowl and mix together.
Place the mixture, in batches, into a food processor and pulse five times for 1-2 seconds each time. Transfer the purée into a large bowl and repeat until all of the vegetables have been puréed.
Cover the bowl with cling film and chill in the fridge for six hours.
Line a clean bowl with a large piece of muslin and spoon in the tomato mixture. Draw the corners of the muslin cloth together and tie up tightly. String the muslin 'bag' up above the bowl to catch the strained juices. Leave to strain overnight. (Do not be tempted to squeeze the muslin.)
Once strained, cover and refrigerate until needed.