Soak mushrooms in warm water until softened. Drain; remove stems and slice caps. Combine mushrooms, ham, bamboo shoots, soy sauce, and chicken stock in large saucepan and bring to boil over high heat. Reduce heat and simmer three minutes. Taste and add salt & pepper if necessary.
Add bean curd and vinegar and bring to a boil again. Stir in dissolved cornstarch and cook, stirring constantly, until soup is slightly thickened.
Slowly add egg, stirring gently. Remove from heat and stir in oil. Ladle into bowls and garnish with green onion.