To make the soy master stock (about 2,25 litres):
Place all the ingredients in a pot with 1,1 litres of water and bring to the boil. Reduce the heat and simmer, covered, for 30 minutes.
To prepare the pork:
Cut the pork into 16 chunks, place in a large pot of cold water, and bring to the boil. Lower the heat and simmer for 20 minutes. Drain in a colander, discarding the water. Return the pork to the pan and pour over the stock. (The meat chunks need to be covered so you may need to add water.) Peel and slice the ginger and add to the pot. Wash the greens and remove the stalks. Bring the stock to the boil, then lower the heat and simmer for about 2 hours, by which time the pork should be moist and tender. (It is important to cook it as slowly as possible to prevent the meat from becoming dry and tough.) When ready, carefully transfer the meat to a plate and leave to cool. As soon as you can handle it, remove the skin and any bones. Return the stock to the boil and skim. Leave it simmering on the hob. To serve, gently warm the pork in a little stock. While the pork is cooking, mix the mustard powder with an equal amount of water in a small bowl and finely slice the spring onions. Blanch the greens in boiling salted water for 2 minutes and refresh in cold water. Squeeze out any excess moisture, but do not compress the leaves too much. Just before serving, gently reheat the greens in a few spoonfuls of stock in a separate pan.
Mound the greens in a warmed serving dish. Arrange the pork on top and dab with mustard. Scatter with sliced spring onion, followed by the bonito. Quickly cover and serve immediately. When you take off the lid, the bonito flakes will have been displaced by the moisture and heat and will be dancing in the air!
Note: The soy master stock is used for Chinese-style braising of beef and chicken. Kikkoman soy sauce is best, but it is too expensive for such a quantity. Instead, buy an imported Chinese brand from an Asian market, where they sell large bottles of soy sauce very cheaply. After you have finished using the stock, allow it to cool, then strain it into a suitable container and refrigerate. It will keep pretty much indefinitely if you sieve it, store in a container with a clean lid, and remember to boil it up twice a week. You can use it again and again, with the addition of a fresh piece of ginger each time.
Ed’s tip: Easier still, simply freeze the stock until needed. Boil and strain before using.