A Bearnaise type sauce that would go well with firm white fish. Incidentally, there are a lot of terrifying rules around making Bearnaise and similar sauces. Never put the pan over direct heat, keep the pan above boiling water, only use clarified butter etc. The best advice of any cook I read about these types of sauces was from Anthony Bourdain and it was quite simple. Never fear the sauce because the sauce will split! So I took that to heart and relax when I am making these trypes of sauces and frequently make them in a pan on a direct heat, occasionally taking them off the heat and keeping an eye on things to make sure the egg base stays smooth and does not start to form scramble, for example. If I have clarified butter to hand, I use it. If not, plain butter seems to work fine.