Preheat oven to 200°C and butter a 23×33cm baking dish.
Cook pasta in salted water until just al dente. Drain.
Make sauce: Melt butter, whisk in flour for 2–3 mins. Slowly add milk, whisking until thickened. Stir in mustard, garlic powder, nutmeg, salt, pepper, and cheese until smooth.
Combine: Mix pasta with cheese sauce and pour into dish.
Topping: Pulse bread into crumbs. Mix with cheese, butter, parsley, paprika, and garlic powder.
Assemble & bake: Top pasta with crumb mix. Bake 20–25 mins until golden and bubbling. Let rest 5 mins before serving.