1.Using muddler, crush finely chopped lemon grass, mint and sugar syrup in a cocktail shaker. Add rum and half the ice cubes; shake vigorously.
2.Strain into a 340ml highball glass filled with remaining ice cubes. Top with soda water.
3.Using the back of a knife, crush the end of the remaining half of lemongrass. Garnish drink with lemongrass, crushed-end down, inside the glass.