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.
Type the ingredients you want to use, then click Go. For better results you can use quotation marks around phrases (e.g. "chicken breast"). Alternatively you can search by chef, programme, cuisine, diet, or dish (e.g. Lasagne).
James Martin is joined by top chef Tom Kitchin for the cooking show.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.