Step 1 In a medium, heavy-bottom saucepan, heat the butter over medium heat until it foams and subsides. Whisk in the flour and stir it well using a wooden spoon, incorporating it into the butter until a dry paste forms (a roux). Reduce the heat and continue to cook and stir, taking care not to let the mixture brown.
Step 2 Meanwhile, in a large saucepan, bring the milk to a simmer, then gradually whisk it into the pan with the roux, continuing to whisk until the mixture is smooth. Season with 1/2 teaspoon salt and a pinch of pepper, or to taste, along with the nutmeg, if using. Continue to cook over medium-low heat, stirring regularly, until the sauce is thick enough to coat the back of a wooden spoon, about 10 minutes. This makes about 4 cups bechamel.