Combine oysters and cornstarch in a bowl and gently rub. Add enough water to cover the oysters, gently rinse, and repeat the process three times until the water runs clear. Drain the oysters.

Cut broccoli into small florets. Place broccoli and 1 tbsp water in a microwave-safe container, cover loosely with plastic wrap, and microwave at 600W for 1 minute. Drain the water. Slice green onion diagonally into 1cm pieces.

Heat sesame oil in a frying pan over medium heat. Add oysters and cook until browned. Add broccoli, green onion, and sake, cover with a lid, and steam for 2–3 minutes.