In a fine sieve set over a bowl let the yogurt drain, its surface covered with plastic wrap, chilled, overnight. In a steamer set over boiling water steam the potatoes, covered, for 10 to 12 minutes, or until they are just tender, and transfer them to another bowl. Toss the potatoes with the lemon juice and let them cool to room temperature. In a small bowl whisk together the yogurt, the dill, the zest, the scallions, the water, and salt and pepper to taste, add the dressing to the potatoes, and combine the salad well. Serves 6. salad 57