Preheat the 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. Set the breadcrumbs aside.

In a medium saucepan over medium heat, warm the milk until steaming but not boiling. Remove from heat and keep nearby.

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

Slowly pour the hot milk into the butter-flour mixture while whisking continuously. Cook, whisking often, until the sauce thickens and bubbles appear, about 3 to 4 minutes.

Remove the pan 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). Mix until smooth and set aside.

Bring a large pot of salted water to a boil. Add the macaroni and cook for 2 to 3 minutes less than the package directions, until the pasta is slightly underdone. Drain in a colander, rinse under cold water, and drain well.

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

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

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