Preheat oven to 375°F. Butter a large casserole dish; set aside. Place the bread in a medium bowl. In a small saucepan over medium heat, melt 1 TBS butter. Pour the melted butter into the bowl with the bread and toss. Set the aside.

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.

Warm the milk in a medium saucepan over medium heat. Melt the remaining 3 tablespoons butter in a high-sided skillet over medium heat. When the butter bubbles, add the flour. Cook, stirring, 1 minute.

While whisking, slowly pour in the hot milk a little at a time into the skillet with butter/flour mixture. Continue cooking, whisking constantly, until the mixture bubbles and becomes thick, 8 to 12 minutes.

Remove the pan from the heat. Stir in salt, nutmeg, black pepper, cayenne pepper, 1.5 cups of the cheddar cheese, and ¾ cup Gruyère (or ⅓ cup Pecorino Romano); Mix until all cheese is melted and smooth and set the cheese sauce aside.

Stir the macaroni into the reserved cheese sauce.

Pour the mixture into the prepared dish. Sprinkle the remaining cheeses and the breadcrumbs over the top.

Bake until golden brown, about 30 minutes (watch as this may take a little longer. you want the bread browned). Transfer the dish to a wire rack for 5 minutes and then serve.