Preheat oven to 400F.

Bring a pot of water to a boil with a large pinch of salt. Once it comes to a boil, cook the pasta for about 6 minutes until al dente.

Meanwhile, melt the butter in a cast iron pan until melted. Add the flour and whisk until combined. Cook for 2 minutes over medium heat. Stream in the milk and whisk until smooth. Stir in the dry mustard, nutmeg, cayenne, salt, and pepper. Stir and cook until it bubbles and the mixture thickens. Finally stir in the cheese. Leave a handful of cheese aside for the top later. Mix until smooth.

Fold in the cooked pasta. Top with the rest of the cheese and bake until the top is golden brown.