Tie the pork tenderloin with kitchen twine to ensure even thickness.

Heat salad oil in a pot over medium heat and sear the pork, rolling it to brown all sides. Remove from the pot.

In the same pot, add water, sake, sugar, soy sauce, green onion, and ginger. Bring to a boil over medium heat.

Return the pork to the pot and simmer over low heat for 8 minutes. Flip the pork, cover, and simmer for another 3 minutes. Turn off the heat and let it rest covered for 15 minutes.

Remove the pork and let it cool slightly before slicing thinly into chashu pieces.

In a frying pan, reduce the cooking liquid over medium heat until thickened.