Soak sliced red onion in ice water for 10 minutes to mellow the flavor, then drain
In small bowl, whisk together olive oil, lime juice, garlic and salt to make dressing
Arrange watermelon, cucumber and drained red onion on serving platter
Drizzle with half the dressing
Top with feta, avocado, mint leaves, and serrano pepper (if using)
Drizzle remaining dressing over top
Serve immediately for best texture