Cut spinach into 3cm lengths, slice onion thinly, and separate shimeji mushrooms from their base.

Cut cod fillets into quarters, place in a microwave-safe dish, sprinkle with sake, cover with plastic wrap, and microwave at 600W for 2 minutes.

Heat olive oil in a pan, sauté shimeji mushrooms and onion over medium heat until softened, then add spinach and cook until wilted.

Sprinkle flour into the pan, stir until no longer powdery, then add milk and seasonings. Stir continuously until the mixture thickens. Add cod and mix well.