Place the lemon juice and sugar in a small saucepan and warm gently until the sugar is fully dissolved. Leave to cool
Place the kuzu and cold water in a sauce pan and stir until dissolved. It will look like a white chalky water
Pour in the lemon syrup and bring to a boil, stirring all the time. Keep the mixture bubbling away and whisk for two minutes until you have a smooth and elastic mixture. Transfer to a baking sheet, cover with clingfilm to stop it getting a skin and leave to fully cool.
Meanwhile whisk the cream until soft peaks
Whisk the set lemon mixture to slacken it and then whisk in 1/4 of the cream until it is fully incorporated. Fold in the remaining cream.
Spoon or pipe the mousse into small ramekins or shot glasses and level off
Place in the fridge for at least an hour to firm up.