Easy Mac and cheese

Adjust your oven rack to the lower-middle position and preheat your broiler. Butter a 13 by 9-inch broiler-safe baking dish and set it aside.
In a large pot bring 4 quarts of water to a boil. Add the macaroni along with 1 tablespoon of salt and cook, stirring often, until the macaroni is tender. Drain the macaroni in a colander and set it aside.
Using the same pot, melt 5 tablespoons of butter over medium-high heat. Whisk in the flour, mustard, black pepper, nutmeg, and 2 teaspoons of salt, and cook for 1 minute to remove the raw flour taste.
Slowly whisk in the milk until the mixture is smooth. Bring the sauce to a boil, then reduce the heat to medium and cook, whisking occasionally, until the sauce thickens to the consistency of heavy cream, about 5 minutes.
Remove the pot from the heat and stir in the cheeses until they are fully melted. Add the drained macaroni to the cheese sauce and cook over medium-low heat, stirring constantly, until the mixture is steaming and heated through, about 6 minutes. Pour the macaroni and cheese mixture into the prepared 13 × 9-inch broiler-safe baking dish.
In a small bowl, combine the breadcrumbs with 2 tablespoons of melted butter and sprinkle the mixture evenly over the top. Place the dish under the broiler and cook until the breadcrumb topping turns a deep golden brown, about 3 to 5 minutes, rotating the dish if necessary for even browning. Remove the dish from the oven and let it rest for 5 minutes before serving.