In A Saucepan, Melt Butter Over Medium Heat. Stir In Flour And Ketchup Until Smooth. Gradually Stir In Milk. Bring To A Boil; Cook And Stir For 2 Minutes. Add Salmon. Prepared Mashed Potatoes According To Package Directions. Spoon Half Into A Greased 11-In. X 7-In. X 2-In. Baking Dish. Top With Salmon Mixture, Remaining Potatoes And Cheese. Bake, Uncovered, At 375 Degrees F For 15-20 Minutes Or Until Heated Through And Cheese Is Melted.