Put potatoes into a saucepan and cover with cold water. Place over high heat, bring to a boil, then turn down heat and simmer until tender, about 20 minutes. Drain and leave to cool for about five minutes.
Meanwhile, in a large serving bowl, whisk together yogurt, olive oil, lemon juice and mustard. Season to taste with black pepper.
Add most of the watercress, chives and green onions, then mix in the still-warm potatoes and toss to combine.
Sprinkle remaining watercress, chives and green onions on top. Divide among four plates.
Tip: You can serve this salad warm, or make it in advance and chill it in the fridge for a couple of hours.
Makes four side salads or two main courses. Per side dish serving: 321 calories, 8 g protein, 8 g fat (1 g saturated fat), 62 g carbohydrates, 9 g fibre, 2 mg cholesterol, 75 mg sodium