Drain the tuna and shred it into small pieces in a bowl. Dice the cheese, cucumber and zucchini (if necessary remove the kernels from the cucumber and zucchini) and add them. Finely chop the onion and mix in with the chopped iceberg lettuce.
Whisk yogurt, mustard, vinegar, oil, salt, pepper and sugar into a dressing and pour over it. Then mix everything together and let it steep in the refrigerator for at least 2 hours.
Before serving, add the chopped chives. Very tasty as dinner or party salad with fresh baguette or ciabatta.