One of the problems of canning tomatoes is the length of time needed to reduce the tomatoes and then having to clean the pans afterwards as the tomatoes tend to stick to the bottom. I have therefore developed a method which solved both these problems.
First cut the tomatoes into pieces, removing the central cores. Transfer to a large, wide frying pan or wok and bring to the boil. Heat the tomatoes until they start to release their juice. Strain off the juice using a colander.
Fill clean jars almost to the top with the tomatoes. Meanwhile, reduce the juice until only a few spoonfuls remain and use it to top up the jars. If you find you have some juice left over, you can always empty the juice out of the tomato jars and reduce it further. Put the lids on the jars but don’t tighten too much: it is important for air to escape as the jars are heated in the next step.
I then heated the filled jars in a bain marie for 15 minutes. I put a trivet on the base of the pan so that the jars were not in direct contact with the heat. Lift out the jars and fully tighten the lids so that the jars will be vacuum sealed once they have cooled