Follow Andy's advice. To make double sure that it's silky smooth, I add just a bit of milk to my roux at a time, whisking it until it's all absorbed. At first, this creates a very, very thick paste. But as I add more milk, again, a little at a time, it begins to thin into a bechemel. This allows me to tailor the thickness of my sauce. By the time the milk is all added, the sauce is the thickness I want. And if I need to thin it more, I just whisk in a bit more milk. The, season with a bit of salt and nutmeg and you have silky-smooth Bechemel sauce.
Also, I have to confess that I wouldn't know how much roux to make for the amount of sauce I want for a particular dish. So I always start out with 3 tbs. each of butter and flour, following the same technique described by Andy to make the roux.
Seeeeeeya; Goodweed of the North
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