Soak 1/2 thinly sliced red onion in cold water. Pat dry 4 cups diced watermelon and 1 seeded and sliced cucumber; toss with a handful of thinly sliced mint. Drain the onion, squeeze dry and add to the salad along with some chopped cashews. Add 1/4 cup olive oil and the juice of 1/2 lemon; season with salt and toss. Top with crumbled goat cheese.