Cut the Pacific saury into thirds. Slice the white part of the green onion thinly and soak in water to remove bitterness, then drain. Thinly slice the core of the green onion.

Mix the seasoning ingredients (oyster sauce, sake, grated ginger) in a tray. Add the thinly sliced green onion and mix well. Marinate the Pacific saury in the mixture for 5 minutes.

Pat the Pacific saury dry with kitchen paper to remove excess marinade. Coat both sides evenly with potato starch.