How to Make It
In a saucepan, combine the lemongrass, water and sugar; bring to a boil, stirring, until the sugar dissolves. Remove from the heat, cover and let steep until cool, about 3 hours. Strain the syrup through a fine-mesh sieve and discard the lemongrass. Stir in the lime juice.
Fill 8 tall glasses with ice. Add 1/2 cup of the syrup to each glass. Top off with ginger beer or seltzer and a shot of rum.