Sprinkle Each Flounder Fillet With Lemon Juice, 1/4 Teaspoon Salt And Pepper. Roll Each Fillet Around A Shrimp And Secure With A Toothpick. Chop Remaining Shrimp; Set Aside. Arrange Rolled Fillets Seam Side Down In An 11-In. X 7-In. X 2-In. Baking Dish Coated With Nonstick Cooking Spray. Cover And Bake At 425 Degrees F For 20-25 Minutes Or Until Fish Flakes Easily With A Fork And Shrimp Turn Pink. Meanwhile, In A Saucepan, Melt Butter Over Medium Heat. Stir In Flour; Gradually Add The Milk, Mustard, White Pepper And Remaining Salt Until Blended. Add Remaining Shrimp. Bring To A Boil; Cook And Stir For 1-2 Minutes Or Until Thickened And Shrimp Turn Pink. Serve Sauce Over Fish.