In a heavy-based saucepan which will hold the heat, simmer together the wine, vinegar, onion or shallot until there is no more than 2 tablespoons of liquid left.
Cut the butter into walnut-sized pieces.
Take the saucepan off the heat and after adding a couple of butter pieces start whisking.
Once the butter begins to melt return to a low heat and, whisking continuously, slowly beat in more butter.
Keep whisking and it will thicken.
Serve with fish.