Boil water in a large pot with a fitted steamer basket. Put the potatoes in the steamer, cover and steam for 15 minutes. Add the green beans and steam for 6 minutes more. Put the cooked potatoes and beans in a dish and set in the refrigerator to cool, or plunge into a bowl of ice water to cool quickly.
Put tuna, olives, onion, tomato and lettuce in a large bowl. Dice the potatoes and green beans, add them to the bowl and toss.
In a small bowl whisk together the oil, lemon juice, mustard, salt and pepper. Pour the dressing over the salad, toss well and serve.