To make the wasabi mayonnaise, place the mayonnaise, wasabi, lime juice and salt in a bowl and stir to combine. Set aside.
Place the water, ginger, lemongrass and salt in a large saucepan over high heat and bring to the boil. Add the scampi and cook for 5 minutes.* Remove, allow them to cool and use a large kitchen knife to cut the scampi in half. Arrange with oysters with black vinegar and tomato on a platter laid with ice and serve with the mayonnaise. Serves 8.
* Scampi are cooked when they float to the surface.