Heat a dutch oven over medium high heat and brown the pork belly on all sides.
Add enough water to cover the pork belly and add the ginger and green onions. Bring to a boil, reduce heat, cover and simmer on low for 2 hours. Pork belly should be tender.
Drain the pork belly. I highly recommend saving the cooking liquid for future use.
Put remaining ingredients in a pot with pork belly and bring to a boil.
Reduce heat and simmer on low for 30 minutes, uncovered. Sauce should be reduced.
Drain and place pork belly pieces on a lined baking sheet. Broil each side for 3-5 minutes, until lightly crisped and caramelized.
Video
Notes
Place the pork belly under the broiler for 5-10 minutes to crisp up the fat. Be careful not to let it burn! It will burn quickly due to the sugar in the braising liquid.
Buy the pork belly with the least amount of fat you can find. It will still have plenty of fat to be nice and tender.
If you don't have sake, you can substitute a dry white wine.
You can substitute chicken stock for the dashi, but the flavor of the dish will not be as good.