Add 1kg of a waxy, fully grown variety of potato to a large saucepan, cover the potatoes with cold water and add a little salt.
Cover with a lid and place on the stove over high heat.
Once boiling, reduce to a very gentle simmer.
Let the potatoes simmer for 20-30 minutes, depending on size.
Meanwhile, finely dice 1 small brown onion (or use half a large white onion if needed).
Add the diced onion to a small saucepan with the beef stock, the white wine vinegar and the Dijon mustard.
Bring the mixture to a boil, then simmer for 1 minute to soften the onion and mellow the vinegar.
Turn off the heat and cover the saucepan to keep the dressing hot.
Check the potatoes - they should pierce easily with a knife, with slight resistance in the centre.
Drain the potatoes, but keep them in the hot pan.
Place the lid back on and let them finish cooking in the residual heat for about 20 minutes, until cool enough to handle.
Once cool enough, scrape off the skins with a knife - they should come away easily.
Cut the peeled potatoes into large, bite-sized pieces.
Place the potatoes into a bowl.
Pour the hot dressing over the warm potatoes.
Mix gently but thoroughly.
Cover the bowl and let sit for 15 minutes to let the flavours mingle.
After 15 minutes, stir again - the dressing should begin to thicken.
Leave for another 15 minutes to allow more dressing absorption.
Stir once more - the potatoes should now be coated with a thickened dressing.
Season with a little salt, cracked black pepper, and a few tablespoons of oil.
Gently mix to emulsify the oil into the dressing.
Serve with chopped chives sprinkled over the top.