Soak shiitake mushrooms in 1 cup of boiling water for 10 minutes till they soften. Drain the mushrooms and reserve the soaked water to be used as stock in the soup. Thinly slice the mushrooms and set aside.
Mix cornstarch in ¼ cup of water to make a slurry and set aside.
Heat oil in a wok or pot, add ginger, garlic, green chilli, celery, spring onion whites and saute on high for a few seconds till fragrant.
Add chicken, mushrooms, carrots and saute for 2-3 minutes till the chicken turns pink.
Add light and dark soy sauce, oyster sauce, vinegar, white pepper powder, salt and saute for 1-2 minutes till the sauces boil and coat the chicken.
Add 3 cups of water & the reserved stock of shiitake mushrooms, bring to a boil and simmer for 5-6 minutes. Add cornstarch slurry slowly to the pot while stirring continuously. Bring the soup to a quick boil.
Slowly add in the whisked egg, swirling with a ladle as you create egg ribbons in the soup. Turn off the heat. Top with spring onion greens, toasted sesame oil and serve.