Halve the red onions and cut into very fine slices. Put into a small bowl with the lime juice- if the limes are very juicy you will only need a couple.
Remove the rind and seeds from the watermelon and cut into 4 cm triangular chunks. Cut the feta into similar sized pieces and put into a large shallow bowl with the watermelon.
Tear off sprigs of parsley and add to the bowl with the chopped mint.Add the onions and their juices and toss all together gently. Season with black pepper- the feta will make it salty enough- and serve.