Lay the lemongrass on a flat work surface and bash using the flat side of a large knife to release the oils. Cut into small pieces (about ½ cm long).
Place the lemongrass in a shaker and press firmly a few times with a muddler. If you don't have a muddler you can use the back end of a wooden spoon or similar.
Clap mint together in your hands to release the scent, then add to the shaker. Add lime, coconut water and a few cubes of ice. Replace the lid and give it a good shake! Strain into an ice filled glass and top with sparkling water. Add rum (if using). Stir gently, garnish with lime and mint, then serve.
