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Bring a large pot of water to a boil for the pasta. Salt the water and cook the pasta to al dente.
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While the pasta cooks, toast the rye bread in a toaster and then butter the hot bread. Coarsely chop the bread and place it in a food processor. Add the parsley and paprika to the bread and process into bread crumbs, reserve.
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Heat a saucepot over medium heat. Add the butter to the pot and melt, whisk in the flour 1 minute and then milk. Season the sauce with salt and pepper, thicken sauce 5 to 6 minutes then stir in the mustard.
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Melt 2/3 of the combined cheeses into the sauce.
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Preheat broiler.
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Drain the pasta and return to the hot pot. Toss the hot pasta with the chopped corned beef and sauerkraut to combine, stir in the cheese sauce and coat. Transfer the mac n cheese to casserole and top with bread crumbs and the remaining cheese.
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Brown under broiler 3 to 4 minutes, serve.