Preheat the oven to 240C/475F/gas mark 9. Puree 2 onions in a food processor with the ras el hanout, salt and pepper. Slice the other 2 onions and lay them on the base of a deep roasting dish big enough to hold the lamb.
Pat the pureed onion mix all over the lamb, top and bottom, and lay it on the bed of onions.
Put the roasting dish uncovered in the upper-middle part of the very hot oven for around 30 minutes, by which point it should have started to brown (it may take another 10 minutes if your oven doesn’t run very hot).
Once browned, pour in enough water to reach halfway up the lamb joint, then cover the dish. Reduce the oven to 200C/400F/gas mark 6 and roast for 1 hour.
Uncover, baste with the liquid at the bottom of the dish, then re-cover and return to the oven. Reduce the heat to 180C/350F/gas mark 4 and cook for 1 hour more. Baste again, re-cover and cook for a further hour.
Basting is important, as it will help to soften the lamb, so don’t skip doing this. After 3½ hours, the meat should be really soft and fall away from the bone easily.
To make the slaw, sprinkle the salt over the shredded cabbage, mix and allow to sit for 10‑15 minutes. Add the lemon juice, parsley and vegetable oil and mix well. Sprinkle with the pomegranate seeds.
Set your table with flatbread, some yoghurt, a bowl of mint leaves to cool, pickled chillies if you want some heat and some cabbage salad for crunch. Bring the lamb to the table and dish it out with a large spoon.