Sear the pork in a non stick frying pan over a medium high heat until lightly browned all over, remove from heat and pat dry excess heat. Pat dry excess fat then place in a shallow flameproof casserole dish. Whisk together soy, sugar, mirin and sake, then pour over the pork. Cover with cold water, add the green parts of the spring onion and the sliced ginger. Place a lid on top then simmer gently without boiling for 2 hours or until tender. Next, remove the pork carefully from the stock and carefully slice into large cubes. Add the pork back to the stock and simmer uncovered until the stock reduces and is rich and sticky. Serve straight away.
TO SERVE: Serve with sticky rice and choi sum, slivers of spring onion and very finely shredded ginger root.