Separate broccoli into small florets, slice the onion thinly, and trim the root of the shimeji mushrooms, breaking them apart by hand.

Pat the salmon fillet dry with a paper towel and season with salt and pepper.

In a microwave-safe container, combine butter and flour. Heat uncovered at 600W for 30 seconds, then mix until smooth. Gradually add milk in three portions, mixing well each time. Add salt, pepper, and torn cheese slices. Cover loosely with plastic wrap and microwave at 600W for 1 minute. Stir until the cheese melts and the sauce thickens.

Spread salad oil thinly on aluminum foil using a paper towel. Layer onion, salmon, shimeji mushrooms, and broccoli on the foil. Drizzle sake over the ingredients and seal the foil tightly.

Place the foil packet in a frying pan. Add water to a depth of 1cm and heat over medium heat. Once boiling, cover with a lid and steam on low-medium heat for about 10 minutes until the salmon is cooked through.