Hard boil eggs. Clean the lettuce, wash and spin dry, cut into bite-sized pieces. Clean and wash the spring onions and cut into thin rings. Quarter tomatoes. Drain the tuna in a colander and roughly chop it up.
Peel and dice the eggs. Mix the vinegar with the mustard, salt, pepper and sugar, fold in the oil. Arrange the salad with tuna and tomatoes on plates and spread the dressing on top.