Place the watermelon cubes in a large shallow bowl.
Halve the avocado(s) lengthwise, remove the pit, and score the flesh into 1/2-inch cubes. Using a spoon, gently scoop the avocado directly into the bowl with the watermelon.
Sprinkle the cilantro or mint and a pinch of fleur de sel over the top.
Add the onion and jalapeño slices.
Spoon the crumbled Greek feta on top.
In a separate bowl, whisk the olive oil, sherry vinegar, and a little more salt to taste.
Pour the dressing over the salad, toss gently to combine, and serve immediately.