Don’t be tempted to mix the sorrel leaves with the potatoes before the spuds are cool, or too long before you intend to eat the dish, as they’ll dull in both colour and taste. The samphire pickling takes just a few minutes, though you could do this up to two days in advance (and if you do that, this becomes a 15–30 minute side).
500–600g baby potatoes, halved
3 tablespoons cold-pressed rapeseed oil
Sea salt and freshly ground black pepper
130ml white wine vinegar
40g caster sugar
2 teaspoons yellow mustard seeds
To pickle the samphire, bring a pan of water to the boil and blanch the samphire for 1 minute. Drain and cool under running water or in an ice bath, drain well again, then put the samphire in a jar or container with a lid into which it fits snugly.
Dry the saucepan, then add the vinegar, sugar and mustard seeds. Bring to a gentle simmer over a low-medium heat to dissolve the sugar. Let this cool for 15 minutes, then pour it over the samphire. Cover and leave at room temperature for at least 1 hour before refrigerating until required (and for up to 2 days). When you need it, drain the samphire through a sieve, reserving the pickling liquor and mustard seeds.
To make the potato salad, put the potatoes in a medium-large pan and cover them with 2–3 times their volume of cold water. Add a good pinch of salt, bring to the boil and simmer gently for 15–20 minutes, or until the potatoes are tender all the way through. Drain and rinse under running water until cool.
Put the potatoes in a large bowl and add the pickled samphire, the mustard seeds from the pickling liquor and the sorrel leaves. Make a dressing by combining 2 tablespoons of the pickling liquor with the oil and plenty of black pepper. Pour this over the potatoes and toss. Check for seasoning and add salt if necessary, though remember the samphire provides occasional salty kicks.