Soak sliced red onion in cold water for 5 to 10 minutes. Drain and pat dry. Set aside.
Combine all dressing ingredients in a jar. Shake well until emulsified. Taste and adjust.
Cut watermelon into 1-inch cubes. Pat dry lightly with paper towel. Slice cucumber into half-moons.
Arrange watermelon and cucumber on a wide serving platter or in a large bowl.
Scatter red onion over the top. Add crumbled feta in chunks throughout the bowl.
Tear mint leaves and scatter generously over everything.
Drizzle with dressing right before serving. Toss very gently. Finish with a pinch of flaky sea salt and extra mint leaves. Serve immediately.