1. Place the potatoes in cold salted water in a medium pan and bring to a boil. Simmer until tender, about 15 minutes, then cool, peel, and quarter.
2. Combine the vinegar and mustard in a small bowl. Add salt and pepper and mix well. Stirring constantly, pour in the oil in a steady stream. Dress the quartered potatoes with about a tablespoon of dressing.
3. Place the lettuce in a large salad bowl. Add salt and pepper and enough dressing to lightly coat the lettuce (all but about 2 tablespoons). Toss, mixing well. Arrange the eggs, onions, tomatoes, radishes, olives, capers, anchovies, and tuna over the lettuce. Sprinkle the eggs, onions, tomatoes, and radishes with salt. Drizzle the salad with any remaining dressing, then toss gently and serve.
Variation: Split 1 large or 2 small baguettes. Pull out and then discard some of the white bread inside and then fill with salad. Set aside for about 20 minutes, then cut into 8 pieces and serve.