American
Cook the macaroni in salted boiling water until just al dente, then drain.
Melt the butter in a saucepan, stir in the flour and cook for a minute to make a roux.
Gradually whisk in the milk and simmer until the sauce thickens and coats the spoon.
Off the heat, stir in most of the grated cheese, the mustard and nutmeg until smooth, then season.
Fold the drained macaroni into the cheese sauce until evenly coated.
For a baked version, transfer to a dish, top with the remaining cheese and grill until golden and bubbling.
Serve hot.