Cut the cucumber in half lengthwise, then, using a spoon, scrape out the seeds. Cut the cucumber halves crosswise into thin half-moon slices. Using a mandoline, cut the onion into 1-inch-long paper-thin slices. Toss the sliced cucumber, onion, tomatoes, watermelon, bell pepper, cheese and mint in a large bowl with enough vinaigrette to coat. Serve immediately.