This recepe i used when i was in cooking classes...used it on several ocasions and it worked very well.
4 fl oz clarified butter
4 oz flour
(roux)
2 qt. milk
1/2 small whole onion peeled
1 whole clove
1 bay leaf
salt/nutmeg/ground white pepper to taste
heat the butter in a heavy saucepot over low heat. add the flour and make a white roux. cool the roux slightly.
in another saucepan scald the milk (while making roux) and when milk forms a little scum on the bottom of the pot its ready. add roux gradually and beat constantly. bring the sauce to a boil stirring constantly and then reduce heat to a simmer. stick the bay leaff to the onion with the clove and add to the sauce. simmer for 15 minutes or if possible 30 or more. stir ocasionally while cooking.
addjust the consistency with more hot milk if necessary.
(the consistancy should be tested on a small glass plate. pour some on the plate and tip the plate up ....run your finger through the middle of the sauce and see how fast it comes back together...if it drips down slowly than its ready to be strained)
so then take a cheese cloth and pour some sauce slowly into it, pressing the sauce through into another container until its all strained.
season very lightly with salt, nutmeg and white pepper but they should not dominate the bechamel sauce.
hope this helps