Shred the cheese and set aside.

Fill a pot with water and bring to a rolling boil.

While the pasta water is coming up to heat, prepare the béchamel sauce by adding the butter to a large sauce pan of med-low heat.

Once the butter is melted and bubbling, add the flour and begin whisking semi-vigorously to fully hydrate all the flour with melted butter. It should look like a thick paste. Continue whisking for 1-2 minutes to cook out the raw-flour taste.

Add about ¼ cup of the milk to the pan and whisk to combine. It will clump significantly - that is okay - just make sure the mixture is homogenous and the milk is fully absorbed. Continue incorporating the milk, ¼ cup at a time, until all the milk is added. This process will ensure the cheese cause will come out silky, not gritty.

Reduce the béchamel to nappe (until thick enough to coat back of spoon) or to a consistency similar to heavy cream (2-3 minutes).

If the water is boiling, set the burner to low to keep the water hot just not boiling. Add the cheese to the béchamel and mix until homogenous and continue to reduce. At this point, salt to taste and add mustard (if using).

While the cheese sauce reduces, bring the water back to a boil and add the pasta. Cook pasta 1-2 minutes less than directions (pasta will continue to cook once added to the cheese sauce).

Reserve some pasta cooking water (~¼ cup) and add the strained pasta to the cheese sauce and mix to coat the pasta. Add the water to thin the sauce if it is too viscous for your preference. Serve immediately.