Melt the butter in a
heavy saucepan over low heat.
Add the onions and cook until translucent.
Stir in the flour with a wire whisk and cook over low heat for about three minutes while stirring constantly. (Do not allow the mixture to brown).
Gradually stir in the milk while whisking
constantly and cook over low heat for about 3-5 more minutes (the sauce should begin to thicken).
Season to taste.