Preheat your oven to 375°F. Butter a 3-quart casserole dish and set 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 evenly. Set the breadcrumbs aside.

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

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

While whisking steadily, slowly pour the hot milk into the butter-flour mixture. Continue cooking and whisking until the sauce thickens and begins to bubble.

Remove the skillet from 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). Stir until the cheese is fully melted and smooth. Set the cheese sauce aside.

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

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

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

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