How to Make It
Preheat the oven to 500°. In a small bowl, combine the butter and garlic. In a large skillet, melt half of the garlic butter. Add the shiitake and oyster mushrooms and cook over high heat, stirring occasionally, until tender and just beginning to brown, about 8 minutes. Season lightly with salt and pepper.
Tear 4 sheets of heavy-duty aluminum foil, about 14 inches square. Spoon the mushrooms onto the foil squares and top with the fish fillets, skin side down. Spread the remaining garlic butter over the fillets and drizzle with the soy sauce and sake. Fold the foil over the fish and seal the edges of the packets. Set them seam side up on a sturdy baking sheet. Bake for 15 minutes, until the fish is tender. Transfer the fish to plates, spoon the mushrooms and juices on top and serve.