Arrange fish fillets full-length in baking dish. Combine butter and lemon juice and pour over fish. Place shallots and wine in a saucepan and boil gently until wine has evaporated. Mix together cream, mustard, salt, and pepper and add to shallots. Bring to a boil and remove from heat. Cool; pour over fish. Sprinkle with scallions. Bake at 425 degrees for 10 minutes or until fish flakes easily.