Place the potatoes in a medium saucepan, and cover them with 1 inch of well-salted cold water. Bring to a boil, and cook for 20 minutes, or until a skewer or paring knife easily pierces a potato. Drain them and plunge them into ice water, letting the potatoes hang out there until they're cool. Drain again, and chill them until needed. The potatoes will keep in the fridge for up to 5 days.
Meanwhile, mix the onion, brine, and 1 teaspoon salt, and set this aside for 15 minutes, during which the onions will soften and mellow slightly. Whisk in the mustard, then the oil and nigella seeds, if using.
Season well with black pepper, and more salt as needed. Taste, and if it's not zippy enough-remember, we want this to season a whole bowl of very neutral, absorbent potatoes-add more pickle brine and/or spicy mustard to taste. Halve the cooled potatoes and add them to the dressing, along with the pickles and dill. Stir to coat.
You can eat this right away, let it harmonize at room temperature for an hour or two, or keep it in the fridge for up to 4 days. (But not if your pota- toes were already 5 days old, okay? Then eat it sooner.)