Layer 3 slices of pork belly slightly overlapping, season with salt and pepper, and coat with potato starch.

Roll the pork slices tightly from one end and repeat for all slices.

Heat salad oil in a pan, place the rolled pork with the seam side down, and cook over medium heat until browned evenly.

Wipe off excess oil, add sake, water, green onion, and ginger. Bring to a boil, then reduce to low heat, cover with a lid, and simmer for 7 minutes. Flip the rolls and simmer for another 8 minutes. Turn off the heat and let cool in the pan.

Mix chopped green onion, salt, pepper, lemon juice, and sesame oil in a bowl to make the negi salt sauce.