Pre-heat the broiler to high or theoven 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 to just shy of al dente. Reserave a mug of the starchy cooking water before draining the pasta.
While the pasta is cooking, heat a medium size skillet over medium heat and add the butter. When the butter melts, add the flour and cook about a minute. Whisk in the milk, reserved starchy cooking water and the 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 the sauce. Pour the pasta into a baking dish and top with the reserved cheese and crumbled rye toast. Place the mac in the oven until the cheese is brown and bubbling.