Cook the elbow noodles according to the package directions, drain, rinse in cool water, and set aside.
While the noodles cook, make the cheese sauce. Heat a large pan over low-medium heat. Once the pan is hot, add the butter and allow it to melt completely. Next, add whisk in the flour, ground mustard, salt, and pepper. Continue to whisk continuously for 2-3 minutes.
Slowly add the heavy cream while whisking to combine. Allow the mixture to thicken, then add the milk. Continue to whisk the milk until the liquid begins to steep and steam. Don’t let the liquid begin to bubble or boil.
Add the cheese a little at a time while whisking. Don’t add additional cheese to the pan until the last cheese has completely melted. After all the cheese has been added, turn off the heat and add the cooked noodles.
Carefully stir to combine. Serve hot, and store any leftovers in the refrigerator for up to 3 days.