Preheat the oven to 375°F (190°C).
In a 10-inch ovenproof skillet, melt 2 tablespoons of the butter over medium heat and add the breadcrumbs. Toast the crumbs in the butter, stirring, until they’re a light golden brown. Season with a pinch of salt and set aside.
Place uncooked pasta in a large bowl and cover with hot tap water. Soak for 10 minutes then drain.
Combine the three cheeses on a board.
In the same skillet, melt the remaining 3 tablespoons of butter over medium-high heat. Add the flour, whisking to combine. Gradually add milk, whisking until smooth. Season with salt, black pepper, cayenne, and nutmeg, and cook until the mixture thickens.
Stir in the drained pasta until evenly coated. Sprinkle the surface with reserved cheese and toasted breadcrumbs.
Transfer the skillet to the oven and bake, uncovered, for 30 minutes. If needed, add another 5 minutes for it to soften.
Let rest for 2 to 3 minutes, then serve.
