Cook the potatoes in a medium saucepan until tender. About 13 minutes. In a small bowl stir together the buttermilk, chives, parsley, mustard, sour cream, vinegar, tarragon, and pepper; set aside. Reserve the boiling water, but remove the potatoes to a colander. Rinse under cold water and set aside. Reduce the heat under the saucepan so the water simmers. Place the egg white in a custard cup. Place a steamer basket in the pan if necessary, and place the cup in the steamer. Cover the pan and steam the egg white until firmly set. About 4 minutes. Allow to cool and finely chop. When the potatoes are cool, slice into 1/4 inch thick slices. Divide the romaine, onion and tomatoes into 4 serving plates. Then arrange the sliced potatoes, ham, and chopped egg white on top. Serve with the dressing on the side.
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