Wash spinach thoroughly, trim roots, and cut into 4cm pieces. Slice the onion thinly.

Mix oysters with cornstarch in a bowl, then rinse gently with water until clear. Pat dry with paper towels.

Heat half the butter (10g) in a pan over medium heat. Add oysters and sear both sides. Sprinkle sake, cover, and steam for 1–2 minutes. Remove oysters.

Wipe the pan clean, add remaining butter (10g), and sauté onions until softened. Add spinach and stir quickly until coated. Sprinkle flour and cook until no raw flour remains. Turn off heat and add milk, salt, and pepper.