Spread tomatoes evenly across prepared trays and roast for 25 minutes or until soft.
Place tomato through a food mill. I usually put the skins through the mill twice to extract as much as I can.
Sterilise bottles and lids and pour passata into hot jars. At this stage place some fresh basil in jars, place lid and tighten.
Line the base of a large stockpot with a tea towel and, working in batches, if necessary, add jars, making sure they don’t touch the sides of the pot or one another. Place a small potato the size of a small fist in the pot.
Pour in enough water to completely cover the jars. Slowly bring to the boil over medium heat. Boil until the potato is cooked. Using a heatproof jug, carefully remove enough water from the pot to allow you to carefully remove jars from water using oven gloves or a thick tea towel. Set aside at room temperature overnight.
Sealed jars can be stored in a cool, dark place for up to 1 year.