Best Mac and Cheese Ever

Serves 12
8 tablespoons (1 stick) unsalted butter, plus more for casserole
6 slices white bread, crusts removed, torn into 1/4- to l/2-inch pieces
5 1/2 cups milk
1/2 cup all-purpose flour
2 teaspoons coarse salt, plus more for water
1/4 teaspoon ground nutmeg
1/4 teaspoon freshly ground black pepper
1/4 teaspoon cayenne pepper
4 1/2 cups (about 18 ounces) grated sharp white cheddar cheese
2 cups (about 8 ounces) grated Gruyère or swiss or 1 1/4 cups (about 5 ounces) grated Pecorino Romano cheese
1 pound elbow macaroni
1. Preheat oven to 375°F. Butter a 3-quart casserole dish; set aside. Place the bread in a medium bowl. In a small saucepan over medium heat, melt 2 tablespoons butter. Pour the melted butter into the bowl with the bread, and toss. Set the breadcrumbs aside
.2. Warm the milk in a medium saucepan over medium heat. Melt the remaining 6 tablespoons butter in a high-sided skillet over medium heat. When the butter bubbles, add the flour. Cook, stirring, 1 minute.
3. While whisking, slowly pour in the hot milk a little at a time to keep mixture smooth. Continue cooking, whisking constantly, until the mixture bubbles and becomes thick, 8 to 12 minutes.
4. Remove the pan from the heat. Stir in salt, nutmeg, black pepper, cayenne pepper, 3 cups cheddar cheese, and 1 1/2 cups Gruyère (or 1 cup Pecorino Romano); set the cheese sauce aside.
5. Cover a large pot of salted water, and bring to a boil. Cook the macaroni until the outside of pasta is cooked and the inside is underdone, 2 to 3 minutes. Transfer the macaroni to a colander, rinse under cold running water, and drain well. Stir the macaroni into the reserved cheese sauce.
6. Pour the mixture into the prepared dish. Sprinkle the remaining 1 1/2 cups cheddar cheese, 1/2 cup Gruyère (or 1/4 cup Pecorino Romano), and the breadcrumbs over the top. Bake until golden brown, about 30 minutes (though we needed a bit more time to get it brown, but your oven may vary). Transfer the dish to a wire rack for 5 minutes; serve.