Southern Mac & Cheese

16 ounces uncooked large elbow macaroni, large shells, or cavatappi pasta
6 tablespoons salted butter
1/3 cup grated yellow onion
2 teaspoons dry mustard
1 teaspoon kosher salt
1/4 teaspoon black pepper
1/8 teaspoon freshly grated nutmeg
1/8 teaspoon cayenne pepper
6 tablespoons all-purpose flour
3 1/2 cups milk
1 3/4 cups heavy cream
2 teaspoons Worcestershire sauce
4 ounces extra-sharp yellow Cheddar cheese, shredded (about 1 cup), plus 4 oz. diced (about 1 cup), divided
4 ounces sharp white Cheddar cheese, shredded, (about 1 cup), plus 4 oz. diced (about 1 cup), divided

Preheat the oven to 350°F:

While the oven is preheating, prepare the baking dish and pasta.
Boil the pasta:

Bring the water to a rolling boil and add salt to taste. Follow the instructions on the pasta packet. We are using elbow macaroni pasta for this recipe and boiled them to al dente.
Make the béchamel sauce:

Melt butter in a large saucepan over medium heat. Add onion, dry mustard, kosher salt, black pepper, nutmeg, and cayenne pepper. Cook, stirring, for 30 seconds.
Add flour, and cook, stirring, until golden, 2 minutes.
Gradually whisk in milk and cream. Bring to a boil, whisking occasionally. Reduce heat to medium-low; simmer, whisking, until slightly thickened, 5 minutes. Stir in Worcestershire sauce.
Add cheese:

Remove saucepan from heat; stir in 3/4 cup each of shredded Cheddar cheeses until melted. (Reserve remaining 1/4 cup each of shredded cheeses.)
Stir in pasta and diced cheeses; pour into a lightly greased 13- x 9-inch (3-quart) baking dish.
Bake macaroni and cheese:

Bake on a rimmed baking sheet in preheated oven until bubbly and golden, 30 minutes.
Remove from oven; increase oven temperature to broil. Sprinkle with reserved shredded cheeses; broil 6 inches from heat until cheeses are melted and golden, about 2 minutes. Remove from oven; cool slightly on a wire rack, about 15 minutes.