Spray a large skillet with cooking spray. Set aside.
Mix Dijon mustard and horseradish together until thoroughly mixed.
Brush melted butter on one side (the outside) of each of the 4 slices of bread. Spread 1/2 of the mustard/horseradish sauce on one side (the inside) of two slices of the bread. Place 1/2 cup of the crushed potato chips on each slice of the bread with the mustard/horseradish on it. Top with three slices of cheese, then the pastrami. Top with the remaining slices of bread, butter on the outside of all slices.
Grill over Medium-High heat (covered) on each side until browned and cheese is melted.