Heat oven to 375 degrees. Butter a 3-quart casserole dish; set aside. Place bread in a medium bowl. In a small saucepan over medium heat, melt 2 tablespoons butter (alternately, melt butter in the microwave). Pour butter into bowl with bread, and toss. Set breadcrumbs aside. Pull out 1 1/2 cups of grated cheddar, and 1/2 cup Gruyere or 1/4 cup Pecorino Romano, and reserve.
Fill a large pot with water; bring to a boil. Add macaroni; cook 2 to 3 minutes less than manufacturer's directions, until the outside of pasta is cooked and the inside is underdone. (Different brands of macaroni cook at different rates; be sure to read the instructions.) Transfer macaroni to a colander, rinse under cold running water, and drain well. Set aside.
In a medium saucepan set over medium heat, heat milk. In the same pot you used for boiling the pasta, melt remaining 6 tablespoons butter over medium heat. When butter bubbles, add flour. Cook, whisking, 1 minute.
While whisking, slowly pour in hot milk. Continue cooking, whisking constantly, until the mixture bubbles and becomes thick.
Remove pan from heat. Stir in salt, nutmeg, black pepper, cayenne pepper, 3 cups cheddar cheese, and 1 1/2 cups Gruyere or 1 cup Pecorino Romano. Stir reserved macaroni into the cheese sauce.
Pour mixture into prepared dish. Sprinkle remaining 1 1/2 cups cheddar cheese, 1/2 cup Gruyere or 1/4 cup Pecorino Romano, and breadcrumbs over top. Bake until browned on top, about 30 minutes. If after 30 minutes, it's not browned to your liking, broil the topping rather than leaving it in the oven, which may cause the pasta to overcook and sauce to dry out. Transfer dish to a wire rack to cool 5 minutes; serve hot.
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