4C milk
5 Tbs butter
10 Tbs flour
Salt
Pepper
Grated Nutmeg
To make 4 cups of bechamel, put 4 cups of milk (whole if you want the richest sauce, but you can also use low-fat...skim really doesn't work well) in a heavy-bottom pan.
Make the roux using 5 Tbs butter + 10 Tbs flour.
Melt the butter and add the flour, cook this mixture over a low flame for a few minutes, then add the cold milk gradually, little by little and mix vigourously until the roux and liquid are amalgamated. Do not add the next ladleful until the mixture is smooth. Continue adding the liquid until the sauce is the thickness you require.
Season with salt and pepper (white if you must have a pure white sauce, but I always just use black) and a little grated nutmeg.
If your roux has lumps once it has boiled, use an an immersion or stick blender. A few seconds of blending with this tool will de-lump your sauce in no time. A basic stick blender doesn't cost much, takes up very little counter or drawer space, and is endlessly useful.
If you don't have a stick blender, you can try to get as many lumps out of as possible by vigorously mixing the sauce with a whisk, and then if you want a perfectly smooth sauce, simply strain it through a sieve.
Cook the roux over low heat for 30 minutes or baked covered with parchment in a 325 oven for 30 minutes