To make the parsley dressing, place the parsley, lemon juice, oil, salt and pepper in a bowl and mix to combine. Set aside.
Place potatoes in a saucepan of cold water over high heat. Bring to the boil and cook for 15 minutes or until tender. Drain, cool slightly and peel. Divide the potatoes, tuna, beans, tomato and olives between plates and spoon over dressing to serve. Serves 4.