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. In a medium bowl, toss the torn bread pieces with the melted butter until evenly coated. Set the breadcrumbs aside.

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

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 flour mixture while whisking continuously. Cook, whisking often, until the sauce thickens and bubbles appear.

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

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

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

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

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.