Here is a very simple & classic sauce that goes well with all sorts of fish, like bass, turbot, dover sole, scallops and halibut. The sauce is rich with just the right amount of acidity, that makes it perfect for seafood.
1 finely chopped shallot
60ml white wine vinegar (this can be replaced by a good dry cider, makes a great sauce for shellfish)
60ml white wine
100g small chunks of butter
Add the shallot, vinegar, white wine and water into a stainless steel pan and simmer until almost all the liquid has gone.
You can at this stage sieve and remove the onion if you like, but I prefer it left in
Reduce the heat and whisk in the butter chunk, by chunk until you have a custard like consistency
Season and keep warm until required