Make the pickled dulse: heat the water and vinegar in a pan and add the sugar and soy, stir until dissolved.
Wash the dulse in cold running water to ensure it is clean and grit-free.
Cover the dulse in the pickle and leave for a minimum of 1 day
Now for the beurre blanc: Place the shallot in a small saucepan with the wine and the vinegar and water. Place over a medium heat and slowly reduce the liquid by at least two-thirds. Strain to remove the shallots.
Put ¾ tbsp reduction in a small saucepan and, over a gentle heat, add a cube of cold butter. Using a small sauce whisk, incorporate the butter into the reduction as vigorously as you can. The sauce requires constant, vigorous whisking.
As the butter melts fully, add another cube, repeating the vigorous whisking. Continue in this manner until the sauce is starting to thicken and at least half the butter has been added. Now the butter can be added a little more quickly, all the while ensuring the saucepan is not getting too hot, as overheating will split the sauce. Make sure the sauce heats up between additions of butter.
Once all the butter has been added, the sauce should be pale, with a creamy consistency. Taste and season with more reduction, lemon juice, salt and ground white pepper. The sauce will keep briefly, but not for long; it is usually made just before serving.
