Melt the butter in a saucepan, add the flour and fry over medium heat for 3 minutes or until the flour turns caramel brown.
Add nutmeg and salt to the saucepan. Reduce heat to low and, while stirring the butter-flour mixture, pour in hot milk in a thin stream. The result should be a uniform liquid with nutmeg inclusions.
Simmer the sauce over low heat for 5-10 minutes or until thickened, whisking constantly. The sauce should be creamy and lump-free.