Cook potatoes in boiling water until just cooked through. In a medium bowl, whisk together the yogurt, mayonnaise, dill, vinegar, pepper and 1/2 teaspoon salt. Pour the dressing over the potatoes and toss thoroughly. (The salad can be prepared to this point a day ahead. Cover and refrigerate, but return it to room temperature before serving.) Stir the onions into the salad and adjust the seasoning before serving.
From Michael McLaughlin's book 'Cooking for the Weekend'. Christie Aspegren, September 93 Round Robin.