Make the dressing: In a small bowl, whisk together the olive oil, lime juice, garlic and salt.
Arrange the watermelon, cucumber, and red onions on a large plate or platter. Drizzle with half the dressing. Top with the feta, avocado, mint, and serrano pepper, if using, and drizzle with remaining dressing. Season to taste and serve.
Notes
This salad is best served fresh and can be adjusted based on preferences for herbs and spiciness.