Cook pasta according to package directions.
Meanwhile, in a separate pot, melt the butter over medium-high heat and whisk in the flour and salt.
Slowly add the milk, whisking constantly. Stir in mustard.
Whisk constantly until the sauce has thickened. (You will know it is ready when the sauce coats the back of a spoon and you can run your finger across the spoon but the sauce remains where your finger did not touch.)
Stir in cheese until melted and remove the pot from heat.
Drain the pasta and return to stock pot. Gently stir in the cheese sauce. Place over medium to medium-low heat until the pasta has reached your preferred consistency.