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, but reduce the cooking time by 1-2 minutes. The pasta should be al dente. Drain well and set aside.
Step 3: In a large saucepan or Dutch oven, melt 1/2 cup unsalted butter over medium heat. Whisk in the flour and cook for 2-3 minutes, whisking constantly, to create a smooth roux. Do not let it brown too much.
Step 4: Gradually whisk in the warm milk, a little at a time, ensuring no lumps form. Continue to whisk until the sauce is smooth and slightly thickened, about 5-7 minutes. Bring to a gentle simmer.
Step 5: Remove the saucepan from the heat. Stir in 1 teaspoon salt, 1/2 teaspoon black pepper, and 1/4 teaspoon ground nutmeg. Add the shredded cheddar and Gruyère (or Fontina) cheeses, stirring until completely melted and smooth. Taste and adjust seasoning as needed.
Step 6: Add the cooked and drained macaroni to the cheese sauce, stirring gently to ensure all the pasta is evenly coated.
Step 7: Pour the macaroni and cheese mixture into the prepared baking dish. If desired, combine panko breadcrumbs with 2 tablespoons of melted butter and sprinkle evenly over the top.
Step 8: Bake for 20-25 minutes, or until bubbly and golden brown on top (if using breadcrumbs). Let it rest for 5-10 minutes before serving for the best consistency.