Debone and remove the skin from the chicken. Cut it into small bite-sized pieces. Slice the vegetables and mushrooms for the stir fry.
If using dried shiitake mushrooms, soak them in water for 2 hours or overnight. In a large bowl, combine shoyu, chicken stock, sake, and sugar to make the sake sauce.
Marinate the chicken in the sake sauce for 30 minutes to 24 hours in the fridge. Boil a large pot of water, cook the bean thread noodles for 5 minutes until translucent, then drain and set aside.
Heat oil in a pot or large wok over medium heat. Add crushed ginger and fry until fragrant. Add the marinated chicken (keep the sauce for later) and stir fry until browned. Add onions, carrots, and celery to the wok, and stir fry for 1-2 minutes.
Add bamboo shoots, watercress, shiitake mushrooms, and the reserved sake sauce. Simmer for 5 minutes, then mix in the noodles and green onions. Serve immediately and enjoy!