Bring the chicken stock to a simmer over a medium heat with the quartered shiitake mushroom. Reduce the heat to low and let the flavours infuse.
Prepare the meatballs
Combine all ingredients for the meatballs together and marinate for ½ hour.Divide the minced pork mixture into equal parcels and form the meatballs with your hand.
Bring a large amount of water to the boil with a pinch of salt.Boil the pork meatballs and the pak choi for 3 minutes or until the meatballs float.
Boil the wheat noodles for 2 minutes.
Serve the soup
Arrange the boiled noodles, meatballs and pak choi in the serving bowl.Cover with the infused stock.
Garnish the soup with the sliced spring onions, celery leaves and fresh bean sprouts.Serve the soup very hot with the chilli paste on the side.
Notes
The flavour of the stock makes or breaks the dish, and it is important to have an intense taste.