Step 1: Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish.
Step 2: Cook the elbow macaroni in a large pot of generously salted boiling water according to package directions, reducing the cooking time by 1-2 minutes so it's slightly al dente. Drain well and set aside.
Step 3: In a large, heavy-bottomed pot or Dutch oven, melt 1/2 cup of unsalted butter over medium heat. Once melted, gradually whisk in the 1/2 cup all-purpose flour to create a roux. Cook, whisking constantly, for 1-2 minutes until it forms a smooth paste and smells slightly nutty (do not brown).
Step 4: Slowly whisk in the 4 cups of warmed whole milk, a little at a time, ensuring no lumps form. Continue whisking until the mixture is smooth and begins to thicken, about 5-7 minutes. It should be thick enough to coat the back of a spoon.
Step 5: Remove the pot from the heat. Stir in 1 teaspoon salt, 1/2 teaspoon black pepper, and 1/4 teaspoon ground nutmeg.
Step 6: Gradually add the shredded cheddar and Gruyère cheeses (or your chosen blend), a handful at a time, whisking constantly until each addition is fully melted and incorporated into a smooth, creamy sauce. Taste and adjust seasonings as needed.
Step 7: Add the drained, cooked macaroni to the cheese sauce and stir gently until every noodle is evenly coated.
Step 8: Pour the macaroni and cheese mixture into the prepared baking dish, spreading it evenly.
Step 9: In a small bowl, combine the 1 cup panko breadcrumbs with the 2 tablespoons melted butter. Sprinkle this mixture evenly over the top of the mac and cheese.
Step 10: Bake for 20-25 minutes, or until the topping is golden brown and the cheese sauce is bubbly around the edges. If desired, you can broil for the last 1-2 minutes for extra crispness (watch carefully to prevent burning).
Step 11: Remove from the oven and let rest for 5-10 minutes before serving. This allows the sauce to set slightly, making it easier to serve.