For the dressing:
1/4 cup chopped shallot
1 1/2 teaspoons Dijon-style mustard
4 tablespoons Sherry vinegar (available at specialty foods shops and some supermarkets)
3/4 cup olive oil
For the salad:
2 pounds boiling potatoes
4 large tomatoes (about 1 1/2 pounds)
1/2 pound mustard greens, stems and center ribs cut out and discarded and the leaves washed, spun dry, and shredded coarse (about 6 cups)
2 tablespoons minced fresh chives, or to taste
Making the dressing:
In a blender blend together the shallot, the mustard, the vinegar, and salt and pepper to taste. Keeping the motor running add the oil, and blend the dressing until it is emulsified. The dressing may be made 2 days in advance and kept chilled in a tightly sealed jar.
Making the salad:
In a large saucepan combine the potatoes with enough water to cover them by 1 inch and simmer them for 20 minutes, or until they are tender. Drain the potatoes, peel them, and let them cool. The potatoes may be prepared up to this point 1 day in advance and kept covered and chilled. Slice the potatoes 1/4 inch thick and arrange them decoratively on a platter with the tomatoes, cut into wedges, and the mustard greens. Pour the dressing over the salad and sprinkle the salad with the chives. Serves 8.