2 cups (8 ounces) elbow macaroni
Salt and pepper
3 tablespoons all-purpose flour
2 teaspoons dry mustard
1/8 teaspoon cayenne
8 ounces mild cheddar cheese, shredded (2 cups)
2 teaspoons cornstarch
4 tablespoons unsalted butter
4 ounces sharp cheddar cheese, shredded (1 cup)
1 cup panko bread crumbs
2 teaspoons Worcestershire
2 large eggs plus 1 large yolk
½ cup finely chopped onion
2 cups water
1 cup heavy cream
1. Adjust oven rack to upper-middle position and heat oven to 375 degrees. Bring 4 quarts water to boil in large saucepan. Add pasta and 1 tablespoon salt and cook 5 minutes. Drain pasta and set aside. Combine flour, mustard, ½ teaspoon salt, ¾ teaspoon pepper, and cayenne in bowl; set aside. Toss mild cheddar with cornstarch in separate bowl; set aside.
2. In now-empty saucepan, melt 1 tablespoon butter over medium-low heat. Add breadcrumbs and cook, stirring often, until golden brown, about 4 minutes. Transfer to plate; set aside.
3. In now-empty saucepan, melt remaining 3 tablespoons butter over medium heat. Add onion and cook until translucent, about 2 minutes. Add flour mixture and cook 2 minutes, stirring constantly. Whisk in water and cream, and bring to boil. Reduce heat to medium-low, and simmer until thickened, about 5 minutes. Remove from heat. Whisk in mild cheddar-cornstarch mixture until smooth. Whisk in eggs, yolk, and Worcestershire. Stir in macaroni.
4. Scrape macaroni and cheese mixture into 13-by-9-inch baking dish. Top with sharp cheddar, followed by toasted breadcrumbs. Bake until set, 18 to 22 minutes. Let rest for 20 minutes. Serve.