The tasty bite of sweet watermelon, salty cheese, and bitter arugula is really delicious. You will need a sweet, ripe melon for the best result. A good way to find that out is to... tap the melon before you buy – if it sounds hollow, then it is ready to eat!
In a bowl combine the thinly sliced onion and vinegar. Let sit for at least 30 minutes. Drain.
Meanwhile, cut the melon into 8-12 slices. Cut away the rind and cut the flesh into 1-inch chunks. Put into a large bowl.
Mix the watermelon with half of the lime juice. Add more lime juice to taste. Let sit in the fridge to marinate and chill.
Just before serving, quickly toss together with the cheese and the mint. Lay arugula down on a serving platter and top with watermelon and cheese mixture, and dot the onions over the top. Serve immediately.