In a large pot, heat the vegetable oil over medium heat. Add the pork belly cubes and brown them on all sides until they develop a nice caramelized color.

Once the pork is browned, add the soy sauce, mirin, sake, sugar, water, ginger, garlic, and half of the chopped green onions to the pot. Stir well to combine.

Bring the mixture to a boil, then reduce the heat to low and cover the pot with a lid. Simmer for about 2.5 to 3 hours, or until the pork becomes tender and easily falls apart when pierced with a fork.

Once the pork is tender, remove it from the pot and set it aside. Skim off any excess fat from the broth and discard the ginger and garlic.

Increase the heat to medium-high and let the broth simmer for another 15-20 minutes, or until it reduces and thickens slightly.

While the broth is reducing, cut the pork belly into smaller pieces if desired. Return the pork to the pot and simmer for an additional 5 minutes to allow the flavors to meld.

Serve the Kakuni hot over steamed rice, garnished with the remaining chopped green onions. You can also enjoy it as a filling for steamed buns or alongside some pickled vegetables for a complete meal.