In large pot of boiling salted water, cook pasta according to package instructions

In a separate saucepan, melt butter over medium high heat

Slowly add flour to melted butter, whisking until completely incorporated

Continue stirring roux until you can no longer smell flour (approximately 40 seconds)

Slowly whisk in room temperature milk until roux is completely mixed in; Reduce heat to medium

Continue stirring sauce until it thickens and coats the back of the spoon; add in salt, pepper, nutmeg and mustard (if using) to taste

Stir in cheese until completely incorporated

Drain pasta and return to original pot

Stir cheese sauce over pasta; Serve immediately