Place the slices of bread down and butter one side of each (if desired). Use only a thin layer of butter.
Using the mustard and cover one piece of the buttered bread with mustard.
Place the pastrami slices on top of the mustard. Then place the other piece of bread on top and slice which ever way you prefer. (In the photos I have opted for diagonal.
Wash and then slice the salad, serving it along side the sandwich.
Notes
A simple recipe which is an alternative to regular beef. You can try using horseradish instead of mustard for a lightly hotter kick. Alternatively try French mustard for a less intense heat.