For the sugar syrup, put the lemongrass and caster sugar in a small pan over a medium heat. Pour over 50ml water. Bring to the boil, then simmer for 5 mins until thickened slightly. Remove from the heat and leave to cool completely and infuse.

Pour the cooled syrup into a cocktail shaker, including the lemongrass chunks. Add the lime juice, gin and a handful of ice. Shake until the outside of the cocktail shaker feels cold.

Strain into chilled glasses filled with ice. Garnish with a lime slice.