Mix the lemongrass, ginger and sugar and stir for about 30 seconds. Add the lemon juice and stir. Then add the grapefruit juice and sake (i.e. Sho Chiku Bai’s Junmai-Shu / Nama sake). Leave for five minutes or up to two hours in the fridge to enhance the flavour. Fill a jug or a cocktail shaker half full of ice. Pour the mixture through a sieve and into the jug. Squeeze the remains in order to extract all the juicy bits. Shake for 10 seconds to cool the mixture and pour through a sieve into two glasses.