Preheat your oven to 350°F. Spray an 11x7 baking dish with nonstick spray. Cut four slices of rye bread into 2-inch cubes.
In a food processor, pulse the remaining slices of rye bread into fine breadcrumbs and set them aside.
Spread the cubed bread across the bottom of the prepared baking dish.
Layer half of the pastrami over the bread.
Top with the drained sauerkraut, chopped pickles, and 1 teaspoon of caraway seeds.
Sprinkle 2 cups of shredded Swiss cheese over the sauerkraut layer.
Add the remaining pastrami on top and press down gently to compact the layers.
Sprinkle the remaining 2 cups of Swiss cheese and 1 teaspoon of caraway seeds over the pastrami.
In a medium bowl, whisk together the milk, eggs, Thousand Island dressing, and mustard.
Pour the mixture evenly over the casserole.
Top with the prepared breadcrumbs.
Cover with foil and bake for 45 minutes.
Remove from the oven, let it rest for 5 minutes, then slice and serve.