In a large saucepan, cook the onion over low heat in the melted butter until the onion is soft but not browned, 5 to 7 minutes.
Stir in the flour.
Remove from the heat and whisk in the milk until thoroughly blended.
Return to medium heat and cook, stirring, until the mixture begins to thicken.
Remove from the heat and stir in the mustard and the 10 ounces of cheddar cheese, the salt, pepper, nutmeg, and hot pepper sauce.
Meanwhile, cook the cavatappi according to package directions until tender, but still firm to the bite, al dente.
Drain, but do not rinse.
Stir immediately into the prepared cheese sauce until well blended.
Adjust the seasonings.
Spoon the mixture into a 9 x 1 3-inch baking dish.
Top with the remaining 2 ounces of cheddar cheese and the Parmigiano-Reggiano.
Refrigerate, if desired.
To serve, let the dish return to room temperature.
Preheat the oven to 400°F.
Bake about 30 minutes, until the mixture is hot, bubbling throughout, and golden.