Scrub the potatoes and carrots, then dice into 1½cm chunks.
Cook in a large pan of boiling salted water for 10 minutes, or until soft, adding the peas for the last 2 minutes.
Meanwhile, to make the dressing, put the yoghurt and mustard into a large bowl with 1 tablespoon of red wine vinegar.
Finely chop and mix in the cornichons and pickled onions.
Add 2 tablespoons of starchy cooking water from the potatoes, then scoop out a few pieces of cooked potato, mash well, and mix through the dressing to make it extra creamy.
Season to perfection.
Drain the veg and leave to steam dry, while you core the apples and dice into 1½cm chunks.
Finely chop most of the chives and stir half into the dressing with the apples, potatoes, peas and carrots.
Sprinkle over the remaining chives and drizzle with 1 tablespoon of extra virgin olive oil, to finish.