1. Marinate chicken with salt, cornflour and rice wine, pepper and sesame oil.
2. Bring chicken stock to boil in a saucepan.
3. Add in the mushrooms and cook for 2 minutes.
4. Add in the chicken and stir to separate the chicken slices.
5. When soup starts to boil, add in the watercress and stir.
6. Cook for 1-2 minutes.
7. Remove and serve in a soup bowl.