With the flat of a heavy knife, crush the lemongrass stalks.
Put the stock in a wok and bring to simmering point. Add lemongrass, salt, pepper, sugar and lemon juice and stir to mix well. Simmer gently for 10 minutes. Remove the lemongrass stalks and discard.
In a small bowl, beat the egg white lightly and combine with the sesame oil. Pour into the stock in a very slow, thin stream. Using chopsticks, pull the egg gently into strands by stirring in a figure of 8 motion.
Turn into a soup tureen or individual bowls, garnish with the coriander and spring onions and serve at once.