Preheat broiler to high or oven to 400F.
Place a large pot of water over high heat and bring it up to a boil to cook the pasta. Once at a boil, add some salt and the pasta, and cook until al dente, according to package directions.
While the pasta is cooking, heat a medium-size skillet over medium heat and add the butter. When the butter melts, add flour and cook about a minute. Whisk in stock, milk and spicy mustard, and bring up to a bubble. Stir in the cheese in a figure-eight motion, reserving enough to sprinkle on the top of the casserole, until it's all melted. Fold in the pastrami and season with salt and pepper to taste.
When the pasta is done, drain it well and combine with sauce. Pour into baking dish and top with reserved cheese and crumbled rye toast. Dot the top of the casserole with a butter and place in oven until cheese is brown and bubbling.
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