Finely slice the scallions and reserve the green part for garnish. Heat the chicken stock until boiling. Add the mushrooms and scallion, cook for 2 to 3 minutes. Add the chicken to the soup. Beat the eggs slightly with a little salt and pepper. Stir the soup well. Pour the beaten eggs slowly and steadily into the soup, stirring constantly so that the eggs remain in shreds. Cook for 1 or 2 minutes, until the egg is set. Add soy sauce and serve in soup bowls, sprinkle with reserved green slices of scallions. Serves 4 - 6