This refreshing salad makes a delicious first course or side dish for warm-weather meals. You can make it up to a day ahead, but add the arugula just before serving for best texture. If you'd like to save time by buying pre-cut watermelon, you'll need about 2 pounds of chunks to equal 6 cups diced.
In a large bowl, combine the watermelon, onion, feta and sunflower seeds. Drizzle with the dressing and toss to coat. Add the arugula and toss again. Season with pepper, to taste, and serve.