½ white onion, finely chopped
100 ml/3½ fl oz white wine vinegar
50 g/2 oz butter, softened and cut into small pieces
1-2 tablespoons chopped fresh parsley
salt and pepper


1.Put the onion and vinegar in a small saucepan. Set over a medium heat and cook until the liquid has reduced by one-third.
2.Process the mixture in a food processor, then pour back into the saucepan
3.Place the saucepan only partially over the heat so that the sauce is heated very slowly. Add the softened butter, a piece at a time, stirring constantly.
4.The sauce will increase in volume and become white and creamy.
5.Stir in the parsley and season with salt and pepper to taste.
6.For poached fish.

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