Heat the oven to 375 degrees. Butter a 1.5-quart casserole dish; set aside. I placed several bread slices in the food processor and processed them until they turned into nice bread crumbs – enough to fill 1 cup. Place 1 cup of bread crumbs in a medium bowl, add 1 tablespoon of melted butter, mix, set aside.
In a medium pan, heat milk until it boils. In a separate pan or a skillet, melt 3 tablespoons of butter, add ¼ cup of flour, cook and stir for about 1 minute to form a roux.
Pour hot milk into the hot flour-butter mixture gradually, while whisking. Continue cooking the mixture, constantly stirring, on medium or high heat until the mixture bubbles and thickens – about 1 or 2 minutes.
Remove the pan from the heat. Add salt, nutmeg, black pepper, 1 and ½ cups cheddar, and ½ cup Gruyere, or pecorino Romano, or Parmesan. While off heat, whisk until all cheese is melted and the cheese sauce has even consistency. Set aside.
Bring a large saucepan with water to boil. Add macaroni, cook 3 minutes less than indicated in manufacturer’s directions, so that pasta is still undercooked inside (the pasta will continue cooking as it bakes in step 6). Transfer macaroni to a colander and rinse under cold running water really well. Running the pasta under cold water will stop the cooking and will wash off the excess starch. Drain pasta well. Add drained macaroni to the cheese sauce.
Pour the pasta with cheese sauce into the prepared dish. Sprinkle the remaining ½ cup cheddar and ¼ cup Gruyere (or pecorino Romano, or Parmesan). Sprinkle the top with breadcrumbs. Bake for 30 minutes, until browned on top. Transfer dish to a wire rack for about 5 minutes, then serve.