Boil in salted water the potatoes for 15' and the green beans for 10'. Drain and rinse with cold water. Drain again. Cut the potatoes in half.
In a deep bowl mix the tuna (drained from the oil), the beans, the potatoes, the eggs quartered, the parsley, the olives, the onions, salt and pepper. Beat the olive oil with the mayonnaise, the vinegar and the garlic and pour the salad.