Wash the potatoes, then halve them or quarter the bigger ones. Place them in a large pan, add a pinch of salt and cover with 2-3 cm of cold water. Bring to the boil, turn the heat down slightly and cook for 8-10 minutes until tender when pierced with a fork. Drain and set aside to let them cool down a little.
Combine the mayonnaise, yoghurt, mustard, chives and scallions, salt and pepper in a large serving bowl and mix well.
Put the potatoes into the serving bowl when they are still a bit warm, and mix them well to coat them with the dressing.
Carefully separate the slices of hot smoked salmon and place 3 on each plate. Add the potato salad and a handful of watercress and lemon wedges.

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