Wash the tomatoes, remove the green stem. Puree with the hand blender. Mix the puree with lemon juice, zest and the preserving sugar in a suitable saucepan and leave covered for 2-3 hours.
Then bring to the boil and cook for 4 minutes, stirring constantly.
Immediately pour the jam into twist-off glasses rinsed with hot water, close tightly and turn upside down.