Mix the XO sauce, soy sauce and sherry together in a cup.
Cook the noodles and plunge them into ice water. Pop the noodles into the bottom of two large bowls and set aside.
Fry the grated ginger and garlic for a couple of seconds in a large deep pan, and then add the pak choi stems, chopped mushrooms and the frozen prawns. Stir fry for 2 minutes.
Mix the cornflour with a little cold water, and then add both that and the sauce mix and and stir fry for another 2 minutes.
Once the frozen prawns are looking nearly done, pour in the hot stock, and throw in the chopped pak choi leaves and the spring onions. Bring to the boil and serve over the cold noodles, getting all the soup in there.