Dissolve flour in cold milk. Set aside.

Melt the butter in a saucepan over low heat. Add in the dissolved flour and stir constantly until mixed and all the lumps are gone, about 2 minutes.

Add salt and pepper. Continue cooking over low heat until sauce begins to thicken, stirring constantly.

Note: For a richer sauce, add 1/4 cup heavy cream. For a thin sauce, add more milk.