Step 1: Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish.
Step 2: Cook the macaroni according to package directions until al dente. Drain well and set aside.
Step 3: In a large, heavy-bottomed pot or Dutch oven, melt the 1/2 cup butter over medium heat.
Step 4: Whisk in the flour and cook for 1-2 minutes, stirring constantly, to create a roux. Do not let it brown too much.
Step 5: Gradually whisk in the warm milk, a little at a time, until the sauce is smooth and thickened, about 5-7 minutes. Bring to a gentle simmer.
Step 6: Remove the pot from the heat. Stir in the salt, black pepper, nutmeg, and Dijon mustard (if using).
Step 7: Add the shredded cheddar and Gruyère cheeses in handfuls, stirring constantly until each addition is fully melted and incorporated before adding more. The sauce should be smooth and creamy.
Step 8: Add the cooked and drained macaroni to the cheese sauce and stir until evenly coated.
Step 9: Pour the macaroni and cheese mixture into the prepared baking dish.
Step 10: If using a topping, combine the panko breadcrumbs with the 2 tablespoons melted butter and sprinkle evenly over the mac and cheese.
Step 11: Bake for 20-25 minutes, or until bubbling and the topping is golden brown. Let it rest for 5-10 minutes before serving.