1. Combine the vinegar, lemon juice, salt, and Worcestershire sauce in a small heatproof bowl. Set the bowl over a saucepan of barely simmering water (a double-boiler arrangement) and warm the mixture, stirring occasionally, for 2 to 3 minutes, until the salt dissolves and the liquid is hot to the touch but not boiling.
2. In a 6- to 8-inch (15 to 20 cm) skillet over medium heat, melt the butter and continue cooking, swirling the pan occasionally, for 3 to 5 minutes, until the solids turn a deep golden-brown and the butter smells nutty — this technique is known as browning butter (also called beurre noisette: butter is cooked past melting until its milk solids caramelise, producing a nutty flavour).
3. Immediately pour the browned butter through a fine-mesh strainer into the warm vinegar mixture, discarding the dark solids left in the strainer. Stir briefly to combine. The finished sauce will be a thin, sharp, buttery emulsion, golden in colour. Serve warm over fish.
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