Place the stock in a saucepan and bring to the boil.
Stir the cornflour and water mixture into the stock. Add the soy sauce, vinegar and spring onions. Return to the boil, then reduce the heat and simmer, stirring occasionally, for about 3 minutes until the stock has thickened slightly.
Pour the beaten eggs gradually into the soup while stirring vigorously until just set in strands.
Remove the pan from the heat, ladle the soup into warmed bowls, drizzle with chilli oil and serve.