Grate the onion and season the sirloin steak with salt and pepper.

Heat salad oil in a frying pan over medium-high heat and cook the steak until browned on one side. Flip and cook to your preferred doneness. Wrap the steak in aluminum foil to rest.

Wipe excess fat from the pan with a paper towel. Add the grated onion and sauce ingredients (sake, mirin, sugar, vinegar, soy sauce) to the pan. Bring to a boil and reduce until the sauce thickens to half its original volume.