Slice cucumbers and onion very thinly on a mandoline or with a knife; rinse onion and drain. Put both in a medium bowl. Peel tough outer layers from lemongrass and smash core with a meat mallet or back of a small heavy saucepan until flattened. Mince core and add to bowl.
Whisk sugar, salt, coriander, pepper, and vinegar together. Pour over cucumber mixture and chill at least 1 hour, stirring gently a couple of times. Serve cold. Keeps, chilled, up to 1 day.
Note: Nutritional analysis is per 1/4-cup serving.