For the potatoes:
For the vinaigrette:
- 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.
- 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.