Boil 4-6 quarts of water in a large pot. Add a tablespoon of kosher salt and pour in uncooked pasta. Cook until al dente, according to package directions.
Drain pasta into a large colander and set aside.
Add butter to the pot over medium-low heat and melt. Stir in flour and cook for several minutes, stirring often, until golden brown.
Meanwhile, pour milk and cream into a microwave safe glass bowl and heat for 2 minutes. (Alternatively, heat milk in a small saucepan on the stove until hot, not boiling). Pour milk/cream mixture into the flour mixture and stir often, bringing to a simmer while it thickens. You will know it's done when the sauce is thick and coats the back of a spoon.
Stir in cream cheese, a pinch of salt and pepper and dry mustard. Whisk until the cream cheese is melted.
Remove from heat and add cheddar cheeses, stirring constantly until completely melted.
Stir pasta into the cheese sauce. Divide into bowls and serve hot.
Notes
Nutritional values are based on serving this as a side dish, with about 10 servings per batch. As a main dish, you will get 6-8 servings.