Grill the watermelon
In a bowl, mix the honey, extra-virgin olive oil, and the lime juice and zest until combined. Rub the mixture all over the sliced watermelon and grill on a hot grill or grill pan for 2 minutes per side, until grill marks appear. Remove from the grill and let cool.
Once cooled, cut the watermelon into 2-inch cubes. Reserve any remaining watermelon scraps for the dressing.
Make the dressing
In a blender, combine 2 cups of the reserved grilled watermelon scraps, honey, apple cider vinegar, pomegranate molasses, and Dijon mustard. Blend until smooth. Reduce the speed to low and slowly drizzle in the olive oil until emulsified. Season with salt to taste.
To plate
Toss the grilled watermelon cubes with the Persian cucumber, grape tomatoes, basil, pistachios, and sliced red onion, adding just enough vinaigrette to lightly coat. Add the crumbled feta and gently fold once more. Season with salt to taste.
Transfer the salad to a serving plate or bowl. Toss the arugula with the remaining tablespoon of olive oil and place it on top of the watermelon salad.