Preheat your griddle, skillet, or panini press over medium heat.
Take two slices of rye bread and spread Russian dressing on one side of each slice.
On one slice of bread (the dressing side), layer thinly sliced pastrami.
Next, add a layer of Swiss cheese slices on top of the pastrami.
On the other slice of bread (also the dressing side), layer sauerkraut.
Carefully place the sauerkraut-covered slice of bread on top of the cheese and pastrami, creating a sandwich.
Spread a thin layer of butter or margarine on the outside of the sandwich.
Place the sandwich on the preheated griddle, skillet, or panini press. Cook for 3-4 minutes on each side, or until the bread is golden brown and the cheese is melted.
Once both sides are nicely grilled and the cheese is melted, remove the sandwich from the heat.
Let it cool for a minute, then slice it in half diagonally.
Serve hot and enjoy your delicious Pastrami Reuben Sandwich!