Cook pasta according to package until al dente; drain well and set aside.

Melt butter in a large pot over medium; whisk in flour and cook 2 minutes until a paste forms.

Slowly whisk in milk constantly about 5-7 minutes until smooth and starting to thicken.

Reduce heat to low; stir in cheddar and fontina until completely melted and smooth.

Whisk in mustard, salt, pepper, and cayenne until incorporated.

Gently stir in pasta to coat every piece; serve hot.