Combine blue cheese and horseradish sauce; spread half of mixture over four slices of bread. Top with beef and, if desired, onion relish. Top with remaining cheese mixture and bread. Brush both sides of bread lightly with butter. Cook sandwiches in a preheated panini maker until golden brown and cheese is melted, about 3 minutes. Use two forks to shred (pull) the roast. If pot roast is not available, substitute 8 thin slices (8 ounces) of deli roast beef. Note: Sandwiches may also be cooked in a preheated ridged grill pan or skillet over medium heat. Place a heavy skillet on top of sandwiches to flatten; cook 3 minutes. Turn, replace skillet and continue to cook 3 to 4 minutes or until golden brown and cheese is melted.