In a large pot of boiling salted water, cook the pasta al dente according to package instructions. Drain, rinse, and set aside.,
STEP 2
Heat the same pot used for the pasta over medium-high heat. Melt butter, then add the flour. Whisk together until they are combined and golden to make a roux, about 2 minutes.,
STEP 3
Slowly add milk, whisking as you pour. Add the mustard, salt, and pepper. Bring the milk mixture to a low boil, then reduce to low and simmer, continuing to whisk until the sauce has thickened about 5 minutes.,
STEP 4
Stir in the cheeses until melted completely. Add the cooked pasta and toss to fully coat.