Cut napa cabbage in half lengthwise, then slice into 1cm wide pieces. Slice green onion diagonally into 1cm pieces.

Place oysters in a bowl with water and gently wash them 2–3 times, changing the water each time. Pat dry.

In a frying pan, layer napa cabbage and green onion, then place oysters on top. Add the seasonings (sake, salt, butter) and cover with a lid. Cook over medium heat until boiling, then reduce to medium-low heat and steam for about 7 minutes until oysters are plump.