Preheat the oven to 450 F. Slice asparagus spears diagonally. Remove the bottom of the Shimeji mushrooms and break apart into small pieces by hand. Salt salmon fillets on both sides.

Take 2 pieces of large aluminum foil squares, and place half of Salmon, asparagus, and Shimeji on each sheet. Sprinkle with Sake and put butter on top. Close up the foil tightly by folding a couple of times in the middle and sides so that they form sealed packets.

Put the packets on a baking sheet, and bake in the oven for 12-15 minutes.

Just before eating, open the foil and top with green onion and Ponzu sauce.