Cut watermelon and cucumber into large pieces – about 1 cm (Grill watermelon quickly on high temperature for extra sweetness and flavour)
Thinly slice sweet onion
Mix together vinegar, olive oil, and mayonnaise. Use twice as much olive oil as vinegar, and twice as much mayonnaise as olive oil. Add salt, pepper, and sugar to taste
Mix dressing into vegetables and let sit in the fridge for 15-20 minutes until watermelon and cucumber begin to sweat. Pour off extra dressing for a nicer presentation
Crumble feta cheese and sprinkle chopped basil on top of the salad and serve