Bring a large pot of water to a boil. Add the pork belly and reduce to a simmer. Cook for 20 minutes, then remove the pork and set aside to cool a little. Discard the water.
While the pork is cooling, combine the sauce ingredients and set aside.
Once the pork is cool enough to handle, slice it into cubes, about 5x5 cm (2x2 inches). Work carefully to try to keep the skin and upper fat from separating from the meat.
Layer the bottom of a large pot with the scallions and ginger, then place the pork skin-side down in the pot. Pour the sauce over the pork (don't worry, it's not supposed to cover the meat). Spread the rock sugar around the pot. Bring to a boil over high heat, then cover and reduce to a simmer.
Cook the pork skin-side down for 45 minutes, then turn the pork over and simmer gently, covered, for another 2.5 hours. Spoon some of the sauce over the pork once in a while during the cooking time.
Once finished cooking, let the cooked pork rest in the sauce for 15 minutes, then spoon off some as much of the rendered fat from the sauce as possible. You can use a turkey baster to help you with this, but try to ensure that you leave most of the sauce.
While the pork is resting, bring a pot of water to a boil. Blanch the choy sum until the stalks are tender, about 2 minutes. Remove, drain, and rinse briefly with cold water to stop them from cooking any further.
Remove the pork from the pot and set aside. Transfer the sauce to a small pot and bring to a boil. Reduce by about 1/3 to make a thicker sauce.
Serve pieces of pork on a bed of choy sum, then spoon some sauce over top. Garnish with thinly sliced scallions, if you like.