Preheat oven to 450F. Spread the bottom of a baking dish with butter. Sprinkle the shallots over the butter. In a small bowl, combine the lemon juice and mushroom slices. Set aside. Place folded fish fillets, seam side down into the baking dish. Season with salt and pepper to taste. Scatter mushroom mixture over the flounder. Add wine and cover with aluminum foil. Bake until fish flakes with a fork, about 10 minutes. Serve with the juices and mushrooms. Makes 6 servings.