Syrup: place the water and raw sugar in a saucepan over a medium low heat. Stir to dissolve the sugar. Remove from the heat and add the lemongrass, ginger and kaffir lime leaves. Leave for a couple of hours or overnight before straining. The syrup will keep in a sterilised bottle for around 2 weeks.
Mojito: place the syrup, lime and rum in a glass and muddle with a muddling stick to release all the juice from the lime. If you don't have a muddling stick you can use the end of a wooden spoon.
Place the mint leaves in your hand and clap them together. Rub the mint leaves around the rim of the glass before popping them in the glass.
Top with ice and sparkling water. Stir to make sure everything is well mixed, and garnish with lime wheels, a mint sprig and a lemongrass stalk.