Season the meat all over with salt and pepper and rub it with olive oil. Set it aside to come up to ambient temperature while you prepare the fire.
Get your fire going and let it build up. The base of the fire should be circular and large enough that when the meat is set over it, it will get heat from all sides.
To suspend the meat, I make a simple spit from some lengths of hazel, a basic contraption easily replicated. The frame consists of two uprights with Y-shaped tops that form a sort of crutch for the crossbar section that sits over the fire.
In this instance, I’ve sharpened one end and pushed it through the leg of lamb. Alternatively, you can hang the meat, using a butcher’s hook (or two or three linked together to form a short chain).
If you set things up as I have here, you can turn the spit periodically, which will ensure the lamb is cooked evenly.
If you hang it, you will need to turn the meat so that it cooks evenly, by relocating the hook in the meat at regular time intervals.
When the flames have burnt down, and you have some really hot embers glowing away, set the lamb over the fire. It should be no more than 60cm above the embers, and the temperature should be such that you can’t hold your hand beneath the meat for any length of time.
While the lamb is cooking, wrap some jacket potatoes in foil and nestle them around the hot embers, turning them occasionally so they too, cook evenly.
Turn the meat periodically as it cooks. Keep feeding the fire and watching it: be on hand to make small adjustments. Cook the meat until it is piping hot throughout; this will take several hours.
I’m being intentionally vague here: there are too many variables involved to suggest exact cooking times.
To get an idea of how it’s getting on, press a skewer or small knife into the thickest part of the meat, leave it there for 30 seconds, then take it out and touch it to your wrist for an instant.
If it feels burning hot, the meat will be ready. When the meat is cooked to medium, it will register an internal temperature of 60°C or thereabouts on a digital probe thermometer.
If you prefer it well done, continue to cook until it registers 72°C. Either way, take it off the fire and transfer it to a warm dish to rest for 20 minutes.
Serve thick slices of lamb with buttery jacket potatoes and a green or tomato salad.