1. Chop the sorrel finely; chop the shallots.
2. Cook the shallots until soft in the white wine, fish stock and vermouth over low heat.
3. Add the cream and simmer until the sauce has thickened slightly.
4. Add the sorrel and cook for 20 seconds.
5. Remove from the heat. Gradually whisk in the cubes of cold butter.
6. Season with salt and pepper and finish with a squeeze of lemon juice.

You can change the texture of the sauce by blending it with an immersion blender for a few seconds.