Light barbecue. (Proper barbecue using charcoal, obviously).
Insert sprigs of rosemary under the skin of the lamb, together with some semi-crushed garlic cloves.
Loosely cover the thinnest parts of the meat with a bit of tin foil.
Insert your meat thermometer. (Stop sniggering at the back, please).
When the fire is ready (coals should be glowing, not flaming), pile the coals down the two sides of the barbecue, leaving a gap in the middle. I also like to add some unlit coals to give the fire a bit more longevity.
In the gap, place a metal container with a 50:50 mix of water and red wine.
Put the grilling grate in place, then put the lamb onto it so that it sits over the metal container and not directly over the fire.
Put the lid on the barbecue. Your barbecue does have a lid, doesn't it? If not, buy a proper barbecue and start again.
Leave it alone for about 2-and-a-half hours. Do not be tempted to keep lifting the lid to take a peek. About half-way through cooking time, check whether the water needs topping up (especially if the barbecue has started smoking, indicating that fat is burning in the container), but apart from that just leave it alone.
Did I mention that you should leave it alone?
When the thermometer says it's done, take it off and rest it for 20 minutes or so.
I've done this at least twice in recent years for Christmas dinner. Doesn't matter if it's raining or snowing. The great thing is that it frees up the oven so you can do yer roasties at the right temperature - and as any fule kno, trying to do yer roasties in the same oven as the meat is a no-no because the meat creates humidity.
I'm just recovering from feeding this to 4 other people this lunchtime.
All said it was extremely nice and tasty and my old man has said that I can do it again... anytime.
The best thing was that I could get everything ready.. And go off flying whilst it cooked.... So I did.. . A lovely session of upsidedownyness, and then bomb back from the airfield in time to do the veg and gravy.... Bliss.