Preheat broiler to high or oven to 400°F.
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.