1 lb of potatoes with the peel1 cube of vegetable stock1 cup of onions peeled and chopped4 cod fillet pieces (about 150 g each) desalted boned and with skin2 tablespoons of olive oil2 tablespoons of chopped chives2 tablespoons of chopped capers2 tablespoons of chopped parsley1 tablespoon of good quality mustard1 tablespoon caster sugar2 tablespoons of white vinegarOlives to garnish (to taste)