preheat oven to 400. in a small bowl, combine dressing and horseradish. using half of the butter, lightly brush half of the bread on one side. place on 15x10x1 inch pan, buttered side down.
spread the dressing mixture evenly on each side of the bread. top with corned beef and then cheese. brush remaining slices of the bread with remaining butter, and place atop cheese, butter side up.
bake for 8 minutes. turn, and press lightly with a spatula. return to oven and bake 5 minutes more or until golden brown. let cool for 5 minutes, cut in half diagonally. serve with small bowls of mustards for dipping, if desired.