Preheat the oven to 400℉ (200℃).
Sprinkle salt on the salmon fillets and let it sit for a while, wipe off excess water.
Remove the stems of maitake mushrooms and cut into florets.
Place onion, salmon, mushrooms, carrots, butter and pour sake on top. Fold the foil over to cover.
Bake in the oven for about 20 minutes, then pour Dashi Ponzu Soy Sauce on top.