First, make the sorbet. Bring the water and sugar to the boil, then add the apples and simmer until they are cooked through and soft. Add the watercress then remove from the heat and leave to rest for 1 minute. Place the mixture in a blender and blend until as smooth as possible, then strain through a fine sieve into a bowl. Stir in the lemon juice and allow to cool in the fridge. Finally, pour the mixture into the bowl of an ice cream maker and freeze according to the manufacturer’s instructions. Transfer to a storage container, cover tightly and freeze for at least half an hour or until ready to serve.