1. Blend the cornflour and water together to form a smooth paste.
2. Put the stock into a large saucepan and bring this to the boil over a moderate heat.
3. Add the sweetcorn, crab, soy sauce and seasoning to the stock. Bring to the boil again and simmer for 4-5 minutes.
4. Remove the simmering stock from the heat and gradually add the blended cornflour, stirring all the time. Return the pan to the heat and bring the soup back to the boil, stirring, until it thickens.
Serve this soup hot.