Arrange fillets in a buttered 13- x 9- x 2-inch baking pan. Pour 1/3 cup wine over top; sprinkle fillets with 1/2 teaspoon salt. Bake at 350° for 15 minutes or until fish flakes easily when tested with a fork. Baste every 5 minutes with wine stock in pan. Transfer fillets to a serving platter, and keep warm. Reserve 1/2 cup wine stock.
Sauté mushrooms in butter in a small skillet until tender; set aside.
Combine reserved wine stock and flour in a large heavy saucepan; beat well with a wire whisk. Place over medium heat, and cook 1 minute, stirring constantly. Gradually add whipping cream; continue to cook over medium heat, stirring constantly, until thickened and bubbly. Remove from heat; stir in remaining wine, sautéed mushrooms, shrimp, remaining salt, and pepper. Pour shrimp-mushroom sauce over fillets, and serve immediately.