Put the potatoes into a saucepan and cover with cold water. Sprinkle in a good pinch of salt and bring to the boil. Cook until tender ( 15-20 minutes) . Drain straight away then set aside until cool enough to handle.
Meanwhile, make the dressing. Pound the garlic, cumin and caraway to a paste in a pestle and mortar. Stir in the lemon juice, harissa and olive oil then season to taste.
Peel the potatoes when they have cooled a little and slice into 1 cm thick rounds. Toss them with the dressing, spring onions and coriander. Check the seasoning before serving.