Wash the potatoes well and cook in salted water for 30-40 minutes, or until soft.
Meanwhile peel the shallot and dice very finely.
Heat the stock, add the diced shallot, bring to the boil briefly and set aside.
Stir the mustard into the stock.
Drain the potatoes, leave to steam dry, then peel and slice.
Put into a bowl with the stock, mix carefully, season with salt and pepper and add nutmeg to taste.
Leave to cool, then mix in the oil and half of the chives.
Serve sprinkled with the rest of the chives.