Cook the macaroni:
Boil the macaroni in a large pot of salted water according to the package instructions. Drain and set aside.

Make the béchamel sauce:
In a saucepan, melt the butter over medium heat. Add the flour all at once and stir well for 1 to 2 minutes to make a roux.
Gradually pour in the milk, whisking constantly to prevent lumps. Cook until the sauce thickens and becomes smooth.
Season with salt, pepper, and a pinch of nutmeg. For an extra creamy sauce, stir in 2 tablespoons of sour cream or crème fraîche at this stage, if desired.

Assemble the gratin:
Preheat your oven to 350°F (180°C).
Rub a baking dish with a halved garlic clove for a subtle flavor, if desired.
Mix the cooked macaroni with the béchamel sauce. Add half of the shredded cheese and stir well.
Pour everything into the baking dish and sprinkle the remaining cheese on top. If you like, add a thin layer of breadcrumbs for an extra crispy topping.