I've made a bechamel sauce and added sherry to it, herbs, etc; the sherry is too overpowering. How can I correct that?
Bechamel sauce is only flour milk and butter. Even with the sherry, it's not a big expense. If you make a mess of one batch it's probably best just to bin it and start a new. Diluting it down with more sauce may work by your probably just going to end up with loads and loads of sauce that you'll never eat.
I really do not think there is much you can do about that. It is probably best to just make another batch of bechamel sauce and mix it with the sauce you already have. The concentration level of the sherry will be halved. Then of course you need to add more of all the other flavorings, herbs and whatnot.
If there is way too much sherry in there, as in 4x what you need, your best bet is to start over. Otherwise, just simmer it for a while (maybe 30 minutes) to boil off the alcohol, which will make the sherry less prevalent. If that doesn't work, you can dilute a bit with extra milk and then more roux.
Creating excess bechamel is not a problem; it is flexible and freezable, so you can always use excess in other dishes. Add cheese and you get mornay for broccoli or other vegetables. Heavy cream makes sauce supreme. Mustard sauce is another good bechamel-derived sauce. You can ALWAYS use excess bechamel or veloute somehow, because of how flexible these mother sauces are.