Cook the green beans in a large pot of boiling water until crisp-tender, stirring occasionally, about 4 minutes. Using a mesh strainer, transfer the green beans to a large bowl of ice water to cool completely. Drain the green beans and pat dry with a towel.
Add 2 teaspoons of salt to the same cooking liquid and bring the liquid to a simmer. Add the potatoes to the simmering liquid and cook until they are just tender but still hold their shape, about 8 to 10 minutes. Transfer the potatoes to the ice water to cool completely. Drain the potatoes and pat dry with a towel.
In a small bowl, whisk the lemon juice, garlic, oil, oregano, 1 teaspoon salt and 3/4 teaspoon pepper. Place the tomatoes, basil and parsley in a large serving bowl. Add the tuna and toss gently to combine. Add the green beans and potatoes and gently combine. Pour the dressing over the salad and toss to coat.
Recipe courtesy of Giada De Laurentiis