1pounddried pastaa short shape like cavatappi, penne, ziti, large elbows preferably with ridges
Bring a large pot of water to a boil, salt it generously, and let the water return to a boil.
Meanwhile, melt the butter in a large heavy saucepan over medium heat. Whisk in the flour and red pepper flakes. Cook, stirring, until the flour mixture is blond in color, about 4 minutes. Gradually whisk in the milk. Increase the heat to medium-high and let come to a simmer, whisking frequently. Reduce the heat to medium-low and let the sauce simmer until it starts to thicken, about 3 minutes. Add the cream, grated cheese, Parmesan, mustard, salt, and black pepper, stirring until everything is smooth. Taste for seasoning, adding more salt and/or black pepper as necessary.
Add the pasta to the boiling water and cook it until barely al dente (follow the package directions but stop a minute before the pasta is completely tender). Set aside 1/2 cup of the pasta cooking water, then drain the pasta.
Whisk the reserved pasta cooking water into the cheese sauce, combining it thoroughly. Add the pasta to the cheese sauce and stir to combine. Stir for another minute or two until the pasta is tender and everything is well incorporated. Serve hot.