Add the remaining ingredients (excluding the ginger beer) and shake.
Strain into a glass filled with more crushed ice and top off with the ginger beer. Garnish with the crushed lemongrass stalk.
Notes: The kalamansi, or sour lime, looks like a green tangerine but has a very sour taste.
Ginger beer has a more pronounced flavor than does ginger ale. Look for it in stores selling food from the Caribbean. Elderflower cordial, though popular in England, might be difficult to source domestically. Look for it on Internet sites selling British food products.
This small fruit, also known as the sour lime, is small and round with slightly flattened ends. It can be picked green or ripe, but the juice stays sour. It is halved and placed alongside dishes of mixed fried noodles and similar one-dish meals and is squeezed over individual servings for a piquant flavor. The juice is used in a cordial concentrate, which is diluted to make a refreshing cold drink.