Place onions in vinegar and sugar for 30 minutes, then drain.
Preheat the barbecue flat-plate or char-grill plate to hot before adding the lamb.
Cook on one side until the first sign of moisture appears. Cook lamb chops for 3-4 minutes on each side for medium or until done to your liking.
Remove lamb from heat, loosely cover with foil and rest lamb for 2 minutes before serving. While the lamb is cooking, add the asparagus to the barbecue and cook for 4-5 minutes until golden and tender.
Mix watermelon with tomatoes, onion and feta, then dress with olive oil. Serve with lamb and asparagus.