To make the bechamel sauce, pour the milk into a saucepan and bring just to a boil over medium heat. Do not let it boil over. Remove the pan from the heat.

Melt the butter in a heavy-bottomed saucepan over medium heat. Whisk in the flour and cook, stirring constantly, for 2 minutes. Add the hot milk in driblets, whisking constantly and taking care to avoid lumps and scorching. When all of the milk has been added, cook the sauce, stirring it frequently with a wooden spoon or silicone spatula, for 10 to 13 minutes, or until it is thick enough to coat the back of the spoon.

Season the bechamel sauce with the salt, pepper to taste, and nutmeg, and remove from the heat. 

The sauce may be stored in a tightly lidded container in the refrigerator for up to 3 days. Reheat it in a saucepan over low heat, adding a splash or two of milk if necessary to loosen it.