Peel the potatoes, cut them into bite-size chunks, and leave them in cold water. Cut the green peppers into 1 cm strips. Keep the potato pieces small and even - a frying pan cooks potato much more slowly than a simmering pot.
Mix the water, oyster sauce, soy sauce, mirin, sake, potato starch, and sugar in a small bowl until the starch has completely dissolved. Do this before you turn on the stove - the sauce thickens within seconds at the end and there is no time to measure later.
Heat sesame oil in a frying pan over medium heat. Add the drained potato and the ground pork at the same time. Break up the meat and turn the potato until it colors, 8 to 10 minutes, until the potato gives with a little resistance left.
Lower the heat slightly, add the green pepper, and cook for about 1 minute only - any longer and it loses its bright color.
Wipe the excess fat out of the pan with a folded paper towel, then turn the heat off completely. Stir the sauce again and pour it in, stirring it through. Adding starch to a hot pan makes it seize into lumps.
Return the pan to a low heat and stir constantly. The sauce turns glossy within about 30 seconds. Take it off as soon as it coats the potato - cooking it longer breaks the starch down and thins the sauce again.
Serve over hot rice. To pack it in a bento, let it cool completely first so condensation does not form in the box.
