Prepare pasta according to package directions for al dente

Melt butter in a large saucepan over medium.

Add onion, mustard, salt, black pepper, nutmeg and cayenne pepper. cook, stirring, 30 seconds.

Add flour, stirring continuously for 30 seconds.
Gradually whisk in milk and cream. Bring to a boil, whisking continuously until thickened.

Stir in Worcestershire sauce. Remove from heat. Stir in 3/4 of cheese until melted. (Reserve remaining 1/4 of cheese for top). Sprinkle remaining cheese on top.

Bake in preheated oven until bubbly and golden. Remove from oven.
Remove from oven; cool slightly on a wire rack, about 15 minutes.