Heat a drizzle of the sesame oil in a large saucepan over a medium heat. Add the chicken thighs, skin-side down, and leave to cook undisturbed for three to five minutes, or until the skin is golden brown. Flip over and cook for a further three minutes.
Push the chicken to one side and add a little more sesame oil to the pan, then add the white parts of the spring onions, along with the ginger and garlic. Fry for two minutes then pour in the cold water, soy sauce and Chinese cooking wine, along with any remaining sesame oil. Bring to the boil, then reduce the heat and simmer for 15 minutes. Taste for seasoning, adding more soy, rice wine or sesame oil as you feel necessary.
Use a slotted spoon to remove the chicken thighs from the soup. Discard the skin (if you don’t like it) and the bones, then use a couple of forks to pull the meat apart.
Add the noodles to the soup and cook for two minutes, then add the pak choi and cook for a further three minutes until tender. Stir the shredded chicken back into the soup, then ladle into four serving bowls. Serve topped with the spring onion greens and a drizzle of chilli oil.