Prepare the sauce for your sandwich by combining brown mustard, mayo and some horseradish to taste.
On a flat surface, lay out 1 slice of bread. Add a layer of green leaf lettuce, then tomato and red onion. Fold each slice of Lebanon bologna and lay onto the sandwich. Add 3 slices of Cooper Cheese. Spread mustard sauce onto 1 side of the remaining bread slice, cap the sandwich and enjoy!