makes about 1 cup (8 fl oz) 250 ml
2 tbsp (30 ml) finely chopped onion or shallot
2 tbsp (30 ml) white wine vinegar
1 cup (225 g) unsalted butter, chilled
salt and white pepper
lemon juice to taste
1. Place the onion or shallot and vinegar in a saucepan. Add 45 ml (3 tbsp) water and bring to the boil. Boil until the liquid is reduced by half.
2. Meanwhile, cut the butter into chunks. Reduce the heat to the lowest setting so that the liquid is below simmering point.
3. Whisking constantly, add a piece of butter. Continue adding the butter, one piece at a time, whisking to melt each piece before adding the next.
4. The sauce should be pale, creamy in appearance and slightly thickened - rather like single cream.
5. Take care not to let the mixture become too hot or it will curdle.
6. Remove the pan from the heat when all the butter has been incorporated.
7. Taste the sauce, then add salt, white pepper and lemon juice to taste.
8. Serve at once with poached fish.