Prepare the watermelon by cutting off both ends, standing it upright, and slicing away the rind. Cut the flesh into uniform one inch cubes and place in a large chilled mixing bowl.

If using red onion, slice it into paper-thin half moons and soak in ice water for ten minutes to mellow the bite. Drain and pat dry before adding to the bowl.

Stack the mint leaves, roll tightly, and slice into thin ribbons. In a small jar or bowl, whisk together olive oil and lime juice, adding a pinch of sea salt and black pepper.

Sprinkle crumbled feta and sliced mint over the watermelon, pour the dressing over, and gently toss to combine without bruising the watermelon.

Transfer to a serving platter and garnish with extra mint leaves. Serve immediately.