Heat a flattop grill or pan on medium heat. Butter one side of each slice of bread. Set the buttered side of bread on the flattop. Add provolone to one slice and Swiss to the other slice. Grill until bread is toasted and cheese has melted, approximately 5 minutes, then remove from the flattop. Meanwhile, place pastrami on the flattop in a pile and the sauerkraut on the flattop in a separate pile. Grill both until warm, 1 1/2 to 2 minutes, then flip and warm the other side. Add mayonnaise to both pieces of bread, then add mustard to one half. Place half the pastrami on one slice and the other half on the other slice. Add sauerkraut to one half and flip the other slice on top. Cut sandwich in half and enjoy!