1Prepare the pork belly: remove moisture with a paper towel, trim uneven parts, prick both sides with a fork, and make shallow slices on the non-fatty side.
2Roll the pork belly tightly with the least fatty side in the middle.
3Tie the pork with kitchen twine, making knots to secure it.
4Boil the pork in a saucepan with enough water to cover it.
5Drain and rinse the pork to remove impurities.
6In a clean saucepan, add chopped ginger, garlic, and green onion stems.
7Return the pork to the saucepan, add water to cover, and simmer for 90 minutes, adding more water if needed and placing a paper towel on top to prevent overflowing.
8Remove the pork and save 500ml of the cooking liquid.
9In a pot, combine cooking sake, mirin, and bring to a simmer to burn off the alcohol. Add sugar and stir until dissolved.
10Add soy sauce, 500ml of water, and the reserved 500ml pork liquid to the sauce.
11Add the pork to the sauce and simmer for 40 minutes, turning every 10 minutes.
12Remove the pork and set aside. Add 2 ladles of sauce to the frying pan with the rendered pork fat.
13Fry the pork until the edges are crispy and caramelized.
14Coat the pork with the thickened sauce and serve.