Start by making the wonton filling. Put all the ingredients, except the prawns, in a bowl and mix together by hand for five minutes. You want to make sure that the pork and seasonings are thoroughly combined. Mix in the chopped prawns until evenly incorporated.
To make each wonton, take a wonton skin and put about one teaspoon of filling in the middle. Wet the edges of the skin with water, then bring the two opposite corners together to form a triangle, trying to remove as much air from the wonton as possible (to prevent it from bursting open later on). Press the edges together to seal. Dust the bottom of the wonton with a tiny bit of cornflour to prevent them from sticking together.
Continue making wontons like this – you should be able to make about three dozen, depending on how generous you are with the filling.
Bring the salted water to the boil in a saucepan, then turn the heat down to medium. Add the wontons one at a time to the water. Don’t rush and don’t crowd the pan, cook in batches if you need to. Once a wonton is cooked, it will float to the surface, around four to five minutes. Scoop out the cooked wonton and place in cold water for 10 seconds. Lift out and set aside. Continue until all your wontons are cooked.
To complete the soup, bring the chicken stock to the boil in a large pot, then turn the heat down to medium. Season with salt, pepper and soy sauce to taste. Add the choi sum or lettuce, then add the cooked wontons, dropping them gently into the soup. Take care not to stir hard as you don’t want to break the delicate wontons. Ladle into bowls. Alternatively, I find it easier to put the wontons into bowls and ladle the stock into the bowls. Garnish each with chopped spring onions and a drop of sesame oil.