1. Preheat your oven to 375 F (190 C). Cook the elbow macaroni according to package directions until al dente. Drain the pasta well and set aside. Do not rinse.

2. In a large, heavy-bottomed pot or Dutch oven, melt 1/2 cup of unsalted butter over medium heat. Once melted, sprinkle in the 1/2 cup of all-purpose flour. Whisk constantly for 1-2 minutes until a smooth paste (roux) forms and cooks out the raw flour taste. It should be light golden in color.

3. Gradually whisk in the warmed whole milk, a little at a time, ensuring no lumps form. Continue whisking as the sauce thickens, which should take about 5-7 minutes. It should be thick enough to coat the back of a spoon.

4. Remove the pot from the heat. Stir in the salt, black pepper, and optional ground nutmeg. Add 3 cups of the shredded sharp cheddar cheese and 1 cup of the shredded Monterey Jack cheese to the sauce. Stir continuously until all the cheese is completely melted and the sauce is smooth and creamy.

5. Add the drained cooked macaroni to the cheese sauce and stir gently until all the pasta is evenly coated. Pour the macaroni and cheese mixture into a 9×13-inch baking dish.

6. In a small bowl, combine the Panko breadcrumbs with 2 tablespoons of melted butter. Stir until the breadcrumbs are evenly coated. Sprinkle the remaining 1 cup of sharp cheddar cheese and 1 cup of Monterey Jack cheese evenly over the macaroni in the baking dish, then top with the buttered Panko breadcrumbs.

7. Bake in the preheated oven for 25-30 minutes, or until the mac and cheese is bubbly around the edges and the topping is golden brown and crispy. Let it rest for 5-10 minutes before serving to allow the sauce to set slightly.