This is one of the very first barbecue dinner party dishes I ever cooked. The green Thai flavours build up layer of fragrant flavour. Most people seem to have got the hang of fast cooking on the BBQ but it's great for slow-cooked melting dishes too. When it comes to cooking big pieces of meat like a shoulder of lamb or a whole chicken or a big brisket of beef on the BBQ, unless you have a cool BBQ which you can control and keep an eye on it's often safest to start off the cooking in the oven and finish it on the BBQ to get that lovely charred crust.
Preheat your oven to 170°C/325°F/gas 3. Place the kaffir lime leaves, ginger, lemon grass, garlic, most of the coriander and chillies into a large pestle and mortar or a food processor and bash or pulse until you have a thick, fragrant, green paste. Stir in the olive oil and the lime juice.
Rub the marinade all over the lamb, making sure you get into all the nooks and crannies. Most of it will fall off during cooking, but it will still give the meat the most fantastic flavour. Season the lamb well and place it in a roasting tray. Cover with tinfoil and pop it in the preheated oven for about an hour.
After half an hour or so, light your BBQ so the flames have a chance to die down nicely. Make sure you have coals piled up high on one side so it's super hot and low on the other side for a more gentle heat to give you some control. Take the lamb out of the oven and cut or break it into a few big chunks, this way it'll be much easier to handle.
Place the lamb on the hot side of the barbie with the tray with all its juices on the cooler side. Squeeze a bit more lime juice into the tray if you like – you want the acidity to be like a mint sauce. Turn the meat regularly, basting it in the juices from the tray as you go. This will give you a nice, dark crust. Give the meat about 10 minutes like this, to build up the colour.
Once your meat is done, remove it to a board, cover loosely with foil and leave to rest. Pour the coconut milk into the tray and allow it to bubble for a couple of minutes until thickened.
Carve your lamb into chunky slices. Serve the sliced lamb with the Thai sauce, sprinkled with chopped chilli and the remaining coriander. An absolute showstopper.