Boil the potato slices for 10 minutes, or until tender but firm. Drain and set aside. Make the vinaigrette by whisking together the lemon juice, salt, pepper, and dry mustard. Add the olive oil in a thin stream, whisking constantly. Add about 1/3 of the vinaigrette, the scallions, and the parsley to the potatoes, tossing to coat them completely, and chill in the refrigerator for at least half an hour. Combine another 1/3 of the vinaigrette with the tuna, tossing gently so as to leave rather large chunks of meat. Line a large salad bowl with the lettuce leaves and place the potatoes on the bottom of the bowl. Arrange the tomato wedges, egg quarters, tuna chunks, olives, anchovies, and green beans in an attractive pattern. Or you may make individual arrangement on dinner plates. The dish may be prepared several hours in advance up to this point and refrigerated. Spoon the remaining vinaigrette over the salad just before serving and sprinkle the parsley on top. Serve with warm French bread. Serves 4 to 6.
