Cut the watermelon into bite-sized cubes and add to a large bowl. If very juicy, drain off excess liquid.
Squeeze the lime juice over the watermelon and drizzle with olive oil. Toss gently to coat.
Add the crumbled feta and most of the mint, and fold gently to keep the watermelon intact.
Season with a pinch of flaky salt and a crack of pepper. Scatter the remaining mint on top.
Serve right away, or chill up to 30 minutes. Best cold and just-tossed.
Notes
Use block feta in brine for the best flavor. Add the mint at the end so it stays vivid, and toss just before serving so the salad stays crisp and bright.