Run the knife around your potatoes then boil them in their skins. Once cooked, remove the skins and slice finely while still warm. Place in a large bowl.
Finely slice the onions and spread over the potatoes.
Pour the hot stock over the potatoes and onions a little at a time - the potatoes should be coated but not swimming in the stock.
Add the oil, mustard and vinegar to taste and mix well. Together with the stock, this will give the impression that the salad is wet. Season with salt, pepper and nutmeg. Serve warm.
Notes
This is an unusual salad because it is normally served warm or at least room temperature. It will look like the potatoes are wet, which is why it is also called wet potato salad.