Watermelon and sweet onion play nicely off each other. A regular, sulfurous onion would overpower the delicate sweetness of the watermelon (though soaking a regular onion in cold water for half an hour would take some of that away).  Melon salsas and other dishes with melon tend to get watery the longer they sit (as with tomato), so make this just before eating, or add the melon just before serving. Beans, especially black beans, make this salsa more substantial and add terrific color, it becomes almost a light meal, but it’s optional. Use as much or as little fresh herb as you like here, but be sure to use fresh as dried just doesn’t work here.

3 c or so seeded, chopped watermelon
3/4 c chopped or finely sliced sweet onion
1 jalapeño pepper, seeded if you like, and minced, optional
3/4 – 1 c cooked (black) beans, rinsed and drained if necessary, optional
2 – 5 T fresh, chopped cilantro, or other, leafy herb such as basil, dill or parsley
1/4 c lime juice or (white) balsamic vinegar
salt and pepper to taste

1. Combine all ingredients. If you like, chill for half an hour or so, no longer than an hour. (Watermelon tastes best cold, I think.)