Whisk the vinegar, honey, 1 teaspoon salt and a generous amount of pepper in medium bowl. Whisking constantly, slowly drizzle in the oil until emulsified. Add the onions to the bowl and marinate for 10 minutes.
Meanwhile, cut the watermelon in half lengthwise, then halve one half. Cut 8 slices of watermelon from each quartered piece and reserve the remaining half of the watermelon for another use. Brush both sides of the watermelon slices with some of the balsamic vinaigrette and sprinkle with salt. Grill, flipping once, until golden grill marks have formed and the watermelon is starting to feel warm and softened, 2 to 3 minutes per side. Transfer to a baking sheet.
Remove the onions from the vinaigrette (reserve the vinaigrette) and grill, flipping once, until softened, 3 to 4 minutes per side. Toss the arugula and mint in the bowl with the reserved vinaigrette to coat well. Lay the grilled watermelon on the bottom of the platter then top with the grilled onions and the greens. Sprinkle with the feta and pistachios. Drizzle with more honey before serving.