Step 1: Melt a stick of butter (100g, 4 oz.) in a saucepan, then add six spoonfuls (50g, 2 oz.) of flour and simmer the resulting roux over medium low heat for a few minutes, taking care not to allow the roux to darken, and remove from the heat.
Step 2: In a separate saucepan, bring a liter (one quart) of milk just barely to a boil.
Step 3: Immediately pour the milk into the pan with the roux. Taking a whisk, whip the roux and milk together vigorously, then put the saucepan back on the heat and bring it up to the boil. It will thicken considerably when it gets to the boiling point.
Step 4: As soon as the sauce begins to boil, lower the heat to low and simmer for about 5-10 minutes, seasoning well with salt and a bit of nutmeg to taste.