Cook the pasta - In a medium saucepan, bring the water to a boil, add the salt and then drop the pasta into the pan. Cook according to the directions on the package but just 1–2 minutes shy of al dente. Drain—don’t rinse.
Make the roux: Melt the butter in your 10-inch skillet over medium heat. Whisk in the flour and cook for about 1 - 2 minutes. It should be pasty.
Add the dairy: Slowly whisk in the milk and cream. Cook, whisking frequently, for 5 - 7 minutes, until slightly thickened. It will be creamy.
Add the Dijon/mustard, garlic, nutmeg, pepper and salt.
Turn off the heat and gradually stir in the cheeses until completely melted and smooth.
Now, stir in the cooked pasta.
Top with Parmesan cheese. If using breadcrumbs, toss them with melted butter and sprinkle over the top.
Bake at 350° F for 20 minutes, until bubbling and golden.
For a more deeply browned top, turn the oven to the broil setting for 1 - 2 minutes at the end.