Slice the onion very thinly and halve the olives. If you don't have a sweet onion and are using red, white or yellow, soak it in ice water for 10-15 minutes to take some of the bite out of it.
Toss the onion, olives, and feta cheese with the watermelon.
Remove the flesh from the rinds of two quarters of a preserved lemon and chop them very finely.
Chop the mint finely as well, and add half of it to the lemon, reserving half to sprinkle over the finished salad.
Add the honey, port, balsamic and EVOO to the lemon and mint and whisk to combine.
Pour the vinaigrette over the salad and toss gently, so as to not crush the melon. Chill well.