Method:
Place potatoes in a medium pot with enough cold water to cover. Bring to a boil then turn down heat and simmer until tender. Drain potatoes and cool slightly, then cut in half.
Meanwhile, cook green beans in boiling salted water until crisp-tender, then rinse under cold running water.
Put tuna, 1/2 cup of the vinaigrette, olives, tomatoes and capers into a large bowl and toss gently. Place a generous handful of greens on each of 4 plates, then arrange potatoes, eggs and green beans on plates. Top with the dressed tuna mixture, then drizzle with remaining 1/2 cup vinaigrette. Garnish with parsley and serve.