Stir the lemon zest into the mayonnaise so it’s ready to serve, then make the salad. Put the sliced onions in a sieve. Boil the kettle, and slowly pour it over the sliced onions – this will make their flavour milder. Top and tail the lemons, then use a small, sharp knife to cut away the pith from each before slicing into very thin rounds. Trim the ends of the cucumbers too, then cut into 4 equal lengths. Use an apple corer to remove the seediest middle parts of the cucumber chunks, then slice thinly and mix with the sliced lemons, onions, oil, vinegar and caster sugar.