1 1⁄2 tsp. kosher salt, plus more to taste
8 oz. hollow pasta, preferably elbow macaroni
Butter, for greasing
7 oz. extra-sharp cheddar, cut into 1⁄2″ cubes (about 1 1⁄2 cups), plus 6 oz. grated (about 2 cups)
2 T. plus 1 tsp. flour
1 1⁄2 tsp. dry mustard
1⁄4 tsp. freshly ground black pepper
1⁄4 tsp. freshly grated nutmeg
1⁄8 tsp. cayenne pepper
2⁄3 cup sour cream
2 eggs, lightly beaten
1 1⁄2 cups half-and-half
1 1⁄2 cups heavy cream
1⁄3 cup grated onion
1 tsp. Worcestershire

Heat oven to 350°. Bring a 4-qt. saucepan of salted water to a boil. Add pasta and cook until cooked halfway through, about 3 minutes. Drain pasta and transfer to a greased 9″ x 13″ baking dish. Stir in the cubed cheddar cheese and set aside. Combine 1 1⁄2 tsp. salt, flour, mustard, black pepper, nutmeg, and cayenne in a large mixing bowl. Add the sour cream and the eggs and whisk until smooth. Whisk in the half-and-half, heavy cream, onions, and Worcestershire. Pour egg mixture over the reserved pasta mixture and stir to combine. Sprinkle the grated cheese evenly over the surface. Bake until the pasta mixture is set around the edges but still a bit loose in the center, about 30 minutes. Let cool for 10 minutes before serving