Combine the peppercorns, lime juice, fish sauce, sugar, oil, ginger and chilli, if using, in a large glass or ceramic dish. Reserve half the dressing in a small bowl. Add the lamb to the remaining dressing in the dish and turn to coat. Set aside for 15 mins to develop the flavours.
Heat a barbecue grill or chargrill on medium-high. Drain the lamb. Cook for 2-3 mins each side for medium or until cooked to your liking. Transfer to a plate. Cover with foil. Set aside for 5 mins to rest.
Arrange cucumber, tomato, onion, coriander and mint on a serving platter. Sprinkle with cashew. Arrange lamb on platter. Serve with reserved dressing.