Heat your oven to 200°C/Fan 180°C/Gas 6. Unwrap the lamb. Pat dry. Set aside and let it get to room temperature.
Peel the garlic cloves. Finely slice 2 cloves and crush the other 2. Use a sharp knife to make incisions all over the lamb and push in the sliced garlic.
Pop the crushed garlic in a pestle and mortar with the ground cumin, coriander seeds and plenty of seasoning. Pound together, then rub all over the lamb, pushing the fragrant paste into any grooves and incisions. Place it into a large heavy-based pan or deep roasting dish. Slide into the oven and cook for 30 mins.
While the lamb is cooking, peel and cut the onions into thick wedges.
After 30 mins, remove the lamb from the oven. Turn the oven down to 160°C/Fan 140°C/Gas 3. Tumble in the onions and pour in 400ml chicken or vegetable stock. Add 2 tbsp cider vinegar, if using. Put a lid on the pot or cover the dish securely with foil.
Put the lamb back in the oven for 2½ hrs. Turn the lamb every 30 mins in the stock.
Carefully lift the lamb out and set it on a plate. Cover with foil and leave to rest for at least 30 mins before serving.
Taste the juices in the dish and add 1-2 tbsp cider vinegar and any seasoning if you think it needs it. Skim off any excess fat. Set over a medium heat and bubble for a few mins so it reduces slightly.
Use two forks to shred the lamb – it will be meltingly soft. Slide the pulled lamb back into the pan with the juices. Toss to coat. Serve with flatbreads, pickles and dips.