Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish.
Cook the elbow macaroni according to package directions until al dente. Drain and set aside.
0.5 pound Elbow macaroni
In a large saucepan or Dutch oven, melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes, until lightly golden, creating a roux.
0.5 pound Elbow macaroni
Gradually whisk in the milk, stirring constantly to prevent lumps. Bring to a gentle simmer and cook until thickened, about 5-7 minutes.
0.5 pound Elbow macaroni
Remove from heat and stir in the shredded cheddar and Gruyere cheeses until melted and smooth. Season with salt, pepper, and nutmeg (if using).
0.5 pound Elbow macaroni
Add the cooked macaroni to the cheese sauce and stir to combine.
0.5 pound Elbow macaroni
Pour the macaroni and cheese mixture into the prepared baking dish.
Bake for 20-25 minutes, or until bubbly and golden brown on top.
Let stand for a few minutes before serving.
Notes
For an extra crispy topping, you can add a layer of breadcrumbs mixed with melted butter before baking.