How to Make It
In a large bowl, toss the watermelon with the red onion. Cover and refrigerate until chilled, about 30 minutes.
Add the arugula to the watermelon. In a small bowl, combine the orange and lime juices and vinegar. Pour the dressing over the salad and season with salt and pepper. Top with the ricotta salata and walnuts; drizzle with the olive oil and serve.