How to Make It
In a small bowl, cover the onion with cold water and ice cubes. Let stand for 30 minutes, then drain and pat dry.
In a medium bowl, toss the watermelon with 1 tablespoon of the vinegar and let stand for 10 minutes.
In a medium bowl, whisk the remaining 1 tablespoon of vinegar with the olive oil and season with salt and pepper. Add the arugula and onion and toss gently. Using a slotted spoon, scatter the watermelon over the salad and top with the goat cheese.