Butter a baking dish. Sprinkle with chopped shallots.
Fold the fillets in half and arrange in the pan. Pour over the wine, fish fumet, and the mussel cooking juice.
Cover with parchment paper and bring to a boil on top of the stove. Bake at 350 degrees .F (180 C) for about 5 to 8 minutes.
Arrange the fish fillets on a serving platter.
Garnish with the mussels, shrimp, and fluted mushrooms.
Prepare the velouté sauce with the cooking liquid and fish stock.
Stir in the cream and simmer for 10 minutes or until sauce is medium thick. Pour over the fish. Garnish with fleurons and serve hot.