Steam the potatoes for about 45 minutes, or until soft when pierced with a small fork.
Tip the water away and sit the cooked potatoes over the still-hot pan beneath, without a lid on top of them, to dry out for 5-10 minutes.
While the potatoes are steaming, prepare the dressing by pouring the rapeseed oil, the liquid from the jars of cornichons and capers, the vinegar and mustard into an empty jar and shaking together.
Get out a large mixing bowl and transfer the cooked potatoes to it, cutting them in half lengthways as you do so.
Grind white (or black) pepper very generously on top.
Give your dressing a good final shake before you pour it over the potatoes and then toss gently but patiently to mix, and leave the potatoes for 10-15 minutes, until just about warm but certainly not hot.
Add the sliced cornichons and capers, plus most of the chopped dill and chives, and toss again, patiently but well.
Taste for seasoning and adjust if necessary.
Turn out into a serving bowl and scatter the rest of your dill and chives on top.
