In a large mixing bowl, combine cubed watermelon, sliced scallions, and optional radishes. Crumble in feta cheese, then add chopped basil and mint. Drizzle with lime juice, olive oil, and honey. Toss gently and season with salt and pepper, then set aside.

In a salad bowl, create a bed of arugula and drizzle with remaining olive oil. Toss gently to coat the leaves.

Mound the watermelon mixture over the arugula, creating an appealing arrangement.

In a separate bowl, mix toasted pistachios with lime zest, red pepper flakes, and remaining olive oil. Stir to coat.

Spoon the pistachio mixture over the salad and garnish with extra herbs and black pepper, if desired.