Put onions in a bowl and add the juice from 2 limes and let them sit for awhile, until the onions turn pink.
Cut up and the watermelon and remove the seeds. They should be rather large. Cut the feta into about the same size chunks. Put both in a large, shallow bowl. Add the chopped mint. Tear off sprigs of parsley and add to the mixture.
Pour the onions and their juices over the watermelon. Add the oil and olives. Gently toss the salad with your hands careful not to mush up the watermelon. Add pepper and taste to see if it needs more lime.
I also cut up a lime so that each person can add more to their serving if they want.