Trim any excess fat off the butterflied lamb leg but make sure to leave a good covering. Score the remaining fat in a criss-cross pattern at 5mm intervals cutting through the fat to the lamb. Place the lamb on a tray and rub with a little vegetable oil all over. Using a microplane or fine grater, grate the white end of the lemongrass over the lamb on both sides then season with a little salt and pepper and massage it all onto the lamb. Cover and refrigerate for at least 30 minutes, or up to overnight if you can.
In a small saucepan mix the vinegar, sugar and water. Place over a medium heat to bring to a simmer and stir to dissolve. Once dissolved remove from the heat and allow to cool. Roughly chop the mint leaves and add to the cooled liquid along with the lime juice and a little salt and pepper. Using a stick blender or blender, blitz the dressing until the mint is fine. Check the seasoning and reserve until needed.
Preheat a BBQ to a medium heat. Place the lamb fat side down on the BBQ. Cook for a couple of minutes or until the fat starts to drip and flare up. Remove the lamb from the heat and allow the flames to dissipate then place the lamb back on the heat on the flesh side and cook for 4-5 minutes. Keep turning the lamb from side to side every 4-5 minutes for about 20 minutes or until it has nice char marks on both sides and the inside is cooked to about medium (if you prefer your lamb a little more cooked continue to cook until you are happy with the degree of doneness). Remove the lamb from the heat and place on a rack to rest for 15 minutes.
Place the toasted coconut, peanuts, chilli and garlic into a large bowl. Reheat the lamb on the BBQ for about 3-4 minutes on each side then place onto a chopping board. Using a sharp knife slice the lamb as finely as you can across the grain. Arrange the lamb nicely on a platter and drizzle with a good amount of the dressing. Toss all of the salad ingredients together then scatter whimsically onto the top of the lamb being careful not to cover it completely. Drizzle with a little more dressing and serve with steamed rice on the side.
If you don’t have a BBQ, to cook the lamb you could follow the same method with a grill pan with a slight variation. Heat the grill pan over a high heat then add the lamb and cook for 4-5 minutes on either side until it has good char marks on it. Transfer to an oven tray and place into a preheated 175 degree oven for 20 minutes turning once. Remove from the oven and place on a rack to rest for 15 minutes.