Arrange the lettuce leaves on a large platter or in a shallow bowl.
In a small jug mix together the oil, vinegar and sugar. Season with salt and pepper and set aside.
Shortly before serving, toss the beans with the spring onions, tomatoes, potatoes, cucumber and a little vinaigrette. Place this on the platter / in the bowl then add small mounds of tuna at strategic intervals.
Ring the platter with halves of hard-boiled eggs, sunny side up, and olives.
Dress with remaining vinaigrette, sprinkle with parsley, and serve.