Fill a medium saucepan with 1 litre cold water. Add a pinch salt and bring to a boil. Top with string beans cut into bite size pieces. Place beans in boiling water and cook for 5 minutes. Carefully drain beans into a colander, then place beans in the saucepan and add cold water to cool beans. Drain again. Place beans in a salad bowl.
Wash lettuce in a bowl of cold water and drain. Arrange on serving plates. Open Solid Whte Tuna can and drain it and place tuna in the salad bowl, breaking the tuna into small pieces. Mix green onion, mayonnaise, sesami oil, soy sauce with salad ingredients and spoon the salad into the lettuce leaves.
