Cut the watermelon into bite-sized cubes. If it is very juicy, place it in a colander for 5 minutes to drain off extra liquid.
Slice the jalapeño in half and remove the seeds and white membrane for mild heat (leave a few seeds if you want it spicier). Finely mince the jalapeño.
In a small bowl, whisk together lemon juice, olive oil, salt, and black pepper.
Add watermelon, jalapeño, and red onion to a large bowl. Pour the dressing over the top and toss gently.
Add crumbled feta cheese and chopped mint. Toss very lightly so the watermelon does not break down.
Serve right away or chill for 15 to 20 minutes before serving.
Notes
Adjust heat by removing jalapeño seeds for mild or adding serrano for extra spice
Best made fresh, but ingredients can be prepped ahead and tossed before serving
Store leftovers in an airtight container up to 1 day; salad releases extra water
Swap feta with cotija or skip cheese; add cucumber, mint, or Tajín for variation