method
1. Put the vinegars and finely chopped onion in a small saucepan ; boil steadily until reduced to 1 tablespoon. Strain and set aside to cool.
2. Add the egg yolks and whisk well with a balloon whisk.
3. Place the bowl over a pan of hot water and continue whisking. Gradually add the softened butter, whisking until the sauce thickens and all the butter has been added.
4. Season and serve.
serving amount
makes 1/2 pint (300 ml)
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