Begin by cutting the watermelon into irregular chunks, aiming for juicy, sticky pieces that are easy to grab with your fork. Place them in a large mixing bowl.
Tear the fresh mint roughly with your hands to release its aromatic oils and bright scent, then add it to the bowl with the watermelon.
Pour the lemon juice over the fruit and mint, then drizzle with olive oil. Gently toss everything together to coat the watermelon evenly with the bright dressing.
Crumb the feta cheese last, gently breaking it apart with your fingers so it stays soft and crumbly; sprinkle it over the salad without fully mixing in yet.
Finish by grinding fresh black pepper over the top and adding a pinch of flaky sea salt if desired for extra flavor. Toss carefully to distribute the feta without breaking it apart too much.
Serve immediately on a plate or in a bowl, allowing the juicy watermelon, salty feta, and fresh mint to shine through in every messy, flavorful forkful.