1. Finely chop celery. Thinly slice mushrooms. Heat oil in a saucepan and stir-fry vegetables for 5 minutes.
2. Cut chicken into small pieces and sprinkle with soy sauce and sherry. Quarter or dice water chestnuts.
3. Add stock to pan with water chestnuts. Bring to the boil, then add chicken and soaking juices. Simmer for 10 minutes. Add bean sprouts and cook for 2 minutes. Taste and adjust seasoning.
4. Beat egg. Remove pan from heat and trickle in beaten egg, stirring constantly. Divide soup between individual soup bowls and serve.