Combine the watermelon and onion in a mixing bowl. Whisk together the oil, vinegar, chopped mint, salt, and pepper. Pour over the watermelon and onion and toss to coat. Add the crumbled feta and toss gently. Serve garnished with fresh mint sprigs if desired.

This salad is best served immediately, but it can be made ahead and chilled in the refrigerator. If the salad develops a lot of liquid after chilling, transfer to a serving dish with a slotted spoon.