Prepare a pan with a steamer basket, add water just to the basket, then bring the water to a rolling boil.

Place the pastrami in the basket, cover and remove from the heat and allow the pastrami to get warmed by the steam. 

In the meantime, prepare the bread. If your bread is sliced thick, just warming it in the oven is enough. I use store-bought Jewish rye and toast it just enough to hold up to the hot meat - very lightly browned.

Slather both slices of bread with your choice of mustard.

Lay two slices of the cheese on one slice of the bread, then top that with the warm pastrami, then a pickle and onion (and tomato if you choose), then the top.

Cut it in half and it's ready to serve. It goes great with any kind of soup, but we like it with tomato soup.