Beat the egg lightly. Cut green parts of escallion into tiny rounds.
Put the stock in a saucepan or wok and bring to a boil, then lower the heat to a minimum.
Slowly pour in the beaten egg, either through the gap of 2 chopsticks or along the back of a fork held about 8 to 10 inches above the saucepan.
Keep moving the chopsticks or fork in a circular motion so that the egg covers the whole surface of the stock.
Remove from the heat and cover for 45 seconds, to allow the egg to set into tender flakes.
Add the salt, sugar and soy sauce, and sprinkle on the escallion. Give the soup 2 or 3 generous stirs.
Transfer to a large warm soup tureen or individual soup bowls and serve immediately.