Kosher salt and freshly ground black pepper,to taste
For serving: extra shredded cheese and crushed saltine crackers,optional
Bring a large pot of salted water to a boil. Add the pasta and cook it just until al dente, according to package instructions. Drain the pasta set aside.
While the pasta is cooking, melt the butter over medium heat in a large pot. Turn the heat to low and whisk in the flour and ground mustard to create a paste. Whisk in milk until smooth. Turn the heat to medium and whisk until the sauce starts to thicken, about 2 minutes.
Stir in the shredded cheese until the cheese is melted and the sauce is smooth.
Add the drained pasta and stir until well coated with the cheese sauce. Season with salt and black pepper, to taste. Garnish with a little shredded cheese and crushed saltine crackers, if desired. Serve immediately.