Wash the courgettes and use a peeler to cut them into thin strips lengthways. Put by.
Mix the mayonnaise with the strained tuna and egg yolks to make the filling.
Finely chop the gherkins, egg whites and olives. Put by.
Arrange the courgette strips alongside each other, overlapping a little bit, and put a generous spoonful of the filling on one side. Roll up the courgette strips to make a cannelloni-shaped roll.
Arrange some grated tomato and dressing on a plate, place the courgette cannelloni on top and finish off with the mixture of finely chopped gherkins, egg whites and olives.