Wash the potatoes thoroughly and cook them in their skins in boiling salted water for about 20 minutes until just tender, or cook them in a pressure cooker for 8 minutes. Then drain, leave to cool slightly and peel the potatoes whilst they are still warm. Cut into thin slices and carefully place them in a bowl.

For the balsamic sauce, place the apple cider vinegar, balsamic vinegar, a little pepper, mustard and Birnel in a bowl and whisk together. Add the oil whilst stirring.

Peel and finely chop the onion.

Heat the stock. Add the sauce, the onion and the mustard to the stock. Bring to the boil and simmer for about 3 minutes.

Allow the sauce to cool slightly, then add it to the potatoes. Leave the salad to stand for at least 1 hour.