Place the mustard seeds, vinegar, white wine and 2 teaspoons of sea salt in a glass bowl and cover with clingfilm. Set aside for 3 days. On the third day, place all the ingredients in a food processor, picking the tarragon leaves first, then blitz until smooth. Add a few splashes of water to loosen, if needed. Taste, adjust the seasoning if necessary, then store in a sterelised jar – it will keep in the fridge for up to a month.