-
1
Bring large pot of salted water to boil; add pasta and cook to al dente; drain and set aside.
-
2
Melt butter over medium heat; whisk in flour, stirring constantly for 1 minute (do not brown).
-
3
Slowly add warm milk while whisking continuously until roux fully incorporates and sauce thickens, 3–4 minutes.
-
4
Remove from heat; stir in cheddar, Gruyère, mustard, cayenne, nutmeg, salt, and pepper until cheeses melt completely.
-
5
Fold cooked pasta into sauce; transfer to buttered gratin dish.
-
6
Combine panko and Parmigiano; sprinkle evenly over top.
-
7
Bake at 200°C for 12–15 minutes until breadcrumb topping is golden and sauce bubbles at edges.
-
8
Rest 3 minutes before serving.