Cook the potatoes in lightly salted water for about 15 minutes or until tender. Steam green beans until just tender and then shock in cold water to stop cooking. While the potatoes are cooking mix together the green onions, most of the parsley, the lemon juice, vinegar and olive oil. Drain the potatoes and place in a serving bowl. Add the beans and dressing and toss well. Gently fold in the tuna, being careful to avoid breaking it up too much. Season with sea salt and ground black pepper and sprinkle with the remaining parsley. Garnish with tomatoes and serve warm or at room temperature.