Peel and halve the red onion and cut into very fine half-moons and put in a small bowl to steep with juice of two limes (more if they are dried up) to bring out the transparent pinkness in the onions and diminish their "onion-ness.".
Remove the rind and seeds (if necessary) from the watermelon, and cut into approximately 1 1/2 inch triangular chunks or use a melon baller. Cut the feta into similar sized pieces and put them both into a large, wide shallow bowl. Add chopped mint, along with sprigs of parsley if desired.
Pour onions, along with their juices, over the salad in a bowl, add olive oil and olives, then using your hands toss the salad gently so that the feta and melon don't lose their shape. Add a grinding of black pepper and taste to see whether the dressing needs more lime.