Start by boiling the pasta in salted water until just shy of al dente—you want it a little undercooked because it’ll finish in the sauce. Reserve 1/2 cup of pasta water before you drain.

Melt butter in a medium saucepan over medium heat. Sprinkle in the flour and whisk continuously for 1–2 minutes; you’re cooking out the raw flour taste. The mixture should bubble and smell slightly toasty, not brown.

Slowly add milk while whisking to avoid lumps. Bring to a gentle simmer; the sauce will start to thicken. If it feels too stiff, loosen with a little reserved pasta water.

Take the pan off the heat and stir in the cream, shredded cheese, mustard, garlic powder, and nutmeg. Cheese should melt into a glossy, smooth sauce—if it looks grainy, warm it briefly on low while stirring, but avoid boiling.

Fold the drained noodles into the sauce. If it seems too thick, add more pasta water a tablespoon at a time until it reaches a saucy consistency that coats the noodles. Taste and season with salt and pepper.

For a crunchy finish, toss breadcrumbs with melted butter, sprinkle over the top, and broil for 1–2 minutes until golden. Otherwise, serve straight from the pot with chopped chives sprinkled on top.