For marinade combine ginger, garlic, turmeric, sugar and rice wine to make a wet paste. Remove lamb meat from the bone and rub in marinade. Add sliced onions and set aside.
Sweet chilli dressing: Whisk sugar and water together, add fish sauce and lime juice, stir to dissolve sugar completely. Add garlic and chilli.
Heat the oil in a large frying pan on a medium heat. Drain the lamb from the marinade and add to the frying pan. Cook for 8 minutes, turning occasionally on all sides. Remove from the frying pan, add onion and cook for one minute or until just softened.
To serve, slice lamb and distributed along with onion in lettuce cups, add coriander and mint and top with the chilli if you desire.