Wash the tomatoes, remove the stems and cut into small pieces. Cook until soft in a saucepan over low heat without adding water and then strain through a fine sieve. It is best to peel the lemon peel in large pieces with a peeler.
Put the tomato paste, preserving sugar and lemon zest in a saucepan, bring everything to the boil while stirring and cook well for at least 3 minutes. Remove the lemon peel.
Pour into prepared jars while still hot and close.