For the dressing, mix all ingredients together and season to taste. Set aside.
Butter the bottom slice of bread then it flip over to start building the sandwich: Layer on 2 slices of cheese, the pastrami, sauerkraut, 2 tablespoons of 18,000 Island Dressing (leftover dressing lasts up to two weeks in the refrigerator!) and the 2 remaining slices of cheese.