Preheat your oven to 375°F. Butter a 3-quart casserole dish and set it aside.

In a small saucepan over medium heat, melt 2 tablespoons of butter. Place the torn bread pieces in a medium bowl, pour the melted butter over them, and toss to coat. Set the breadcrumbs aside.

In a medium saucepan over medium heat, warm the milk until it's hot but not boiling.

In a large, high-sided skillet, melt the remaining 6 tablespoons of butter over medium heat. Once the butter bubbles, whisk in the flour and cook for 1 minute, stirring constantly.

Slowly pour the hot milk into the butter-flour mixture while whisking continuously. Keep whisking until the sauce bubbles and thickens.

Remove the skillet from the heat. Stir in the salt, nutmeg, black pepper, cayenne, 3 cups of the cheddar cheese, and 1 1/2 cups of the Gruyere (or 1 cup of Pecorino Romano) until melted and smooth. Set the cheese sauce aside.

Fill a large pot with water and bring it to a boil. Add the macaroni and cook for 2 to 3 minutes less than the package directions, until the pasta is just barely tender on the outside but still firm inside. Drain the pasta in a colander, rinse it under cold running water, and let it drain well.

Stir the drained macaroni into the reserved cheese sauce until evenly coated.

Pour the macaroni mixture into the prepared casserole dish. Sprinkle the remaining 1 1/2 cups of cheddar cheese and 1/2 cup of Gruyere (or 1/4 cup Pecorino Romano) over the top, then scatter the buttered breadcrumbs evenly on top.

Bake until the top is golden brown and bubbling, about 30 minutes. Let the dish cool on a wire rack for 5 minutes before serving hot.