Boil eggs hard in about 10 minutes, rinse with cold water, peel and slice. Clean and wash the tomatoes and cucumber, also cut into slices. Drain the corn. Peel onion and cut into thin rings. Clean and quarter the lettuce and cut out the stalk. Cut lettuce into fine strips, wash and drain.
Mix vinegar, salt, pepper and sugar. Mix the salad ingredients and vinegar and leave to stand for a while. For the sauce drain the tuna, puree it and mix it with yoghurt. Season with salt, pepper and lemon juice. Pour over the salad. Serve garnished with lemon wheels and herbs