In a large saucepan combine the potatoes with enough water to cover them by 1 inch, bring the water to a boil, and boil the potatoes for 15 to 20 minutes, or until they are tender. While the potatoes are cooking, in a blender blend the tofu, the lemon juice, the tarragon, and 2 tablespoons water until the mixture is just combined, with the motor running add the oil in a stream and blend the "mayonnaise" until it is emulsified. Drain the potatoes, quarter them, and in a bowl toss them with the onion, the parsley, the mayonnaise, and salt and pepper to taste.
