A flattop grill or pan should be heated to medium. Each bread slice should have butter on one side.
Place the bread's buttered side down on the tabletop. Swiss and provolone cheeses should be added to each slice separately.
Grill the bread for about 5 minutes, or until it is toasted and the cheese is melted, then remove from the flattop. Meanwhile, arrange the sauerkraut in a separate mound from the pastrami on the flattop.
Grill both for 1 1/2 to 2 minutes until warm, then turn them over and reheat the other side. Spread mayonnaise on both pieces of bread before spreading mustard on one.
One slice should have half the pastrami, and the other should have the other half.
The other slice should be placed on top of the sauerkraut-topped half. Enjoy sandwich by cutting it in half!