Combine The Drained Tuna, Mayonnaise, Horseradish Sauce, And Pickles In A Small Bowl Until Evenly Mixed. Place A Slice Of Swiss Cheese Onto 2 Slices Of Bread, And Top With A Leaf Of Lettuce. Spread The Tuna Mixture Onto The Lettuce Leaves. Top With The Tomato And Red Onion Slices, And Finish With The Remaining Slices Of Bread.