Mix together the sake, water, soy sauce, sugar, green onions and ginger in a large pot.
Halve or quarter the pork belly into large, manageable pieces.
Transfer the pork belly pieces to the pot. Bring to just under a boil, but do not boil. Reduce the heat to a simmer poach slowly on the stove (alternatively, you can use an oven at 300 degrees F) until tender, about 2 hours 30 minutes.
Check for tenderness by piercing the middle of each piece. When tender, allow to cool in the liquid for 2 hours off the heat, then transfer to the refrigerator to completely rest overnight in the liquid.
The next day, remove the pork belly from the liquid and cut into 1/2-inch-thick slices. Warm the pork belly with a bit of sauce and serve as a topping for Spicy Miso Ramen with the various garnishes. Serve 2 slices per person.