Cut the slice of bread in half. Over medium-high heat in a preheated pan, toast on both sides until golden brown. Remove the bread from the heat. Keep the pan on the burner.
Turn the heat down to medium. Sprinkle a bit of water in the pan. Add the pastrami slices and cover the pan with the lid. After 20-30 seconds, turn the meat to the other side. Remove it from the heat when it's steaming hot and right before it starts to shrivel.
Turn off the heat. Working on the kitchen countertop, smear a tablespoon of mustard (or Russian dressing) on one side of the halved bread. Top one side with sauerkraut (or coleslaw). Top the other with the pastrami slices.