Place fish in a 13 x 9-inch baking dish coated with cooking spray.
Sprinkle with shallots, wine, salt, and pepper; cover loosely with foil.
Bake at 350 for 25 minutes.
Let fish cool to room temperature.
Remove fish from dish into a bowl, reserving cooking liquid. Shred fish with 2 forks; set aside. Strain cooking liquid through a sieve into a bowl; discard solids.
Combine the cooking liquid, mushrooms, clam juice, and lemon juice in a large nonstick skillet. Cover and cook over medium-high heat for 3 minutes or until mushrooms are tender. Stir mushroom mixture and fish into Bchamel Sauce; pour bchamel mixture into six gratin dishes coated with cooking spray.
Place shrimp on bchamel mixture, and sprinkle with breadcrumbs.
Drizzle with margarine. Broil on second shelf 5 minutes or until shrimp are done.