Spread Flour Slices Of Bread With Mayonnaise. Layer Each With Ham, Cheese And Onion. Spread Horseradish Sauce On Remaining Bread; Place Bread With Horseradish Side Down Over Onion. Spread Butter On The Outside Of Each Sandwich. In A Skillet Over Medium Heat, Toast Sandwiches Until Bread Is Lightly Browned On Both Sides And Cheese Is Melted.