Place the sugar, lemon rind and juice, lemongrass stalk in a pan with 150ml/¼ pint water.
Heat gently, stirring until the sugar dissolves then simmer gently for five minutes then allow to cool.
Strain the liquid into a rigid container then stir in the soda water.
Transfer to the freezer for at least two hours, whisking every 20 minutes to stop crystals forming.
Serve in glasses.