Put the garlic, cinnamon, cumin, oregano and lemon in a blender with 1 tsp salt and 2 tsp black pepper. Blend until smooth.
Put the lamb in a large roasting tin and tip the marinade over the top. Massage all over the lamb and leave for 24 hrs in the fridge, or up to 2 days.
Remove lamb from the fridge 1 hr before cooking. Heat oven to 160C/140C fan/ gas 3. Scatter the onions around the lamb, tucking some underneath, then pour over the pomegranate juice. Cover the lamb with foil and cook for 4 hours.
Remove the foil, carefully pour the meaty pomegranate juices from the tin into a large saucepan and add the honey. Increase oven to 220C/200C fan/gas 7, re-cover the lamb and continue cooking for 30 minutes Meanwhile, bubble the cooking liquid over a high heat until reduced, thick and syrupy – this may take up to 30 minutes.
Pour the sticky pomegranate glaze over the lamb and onions and return to the oven for another 30 mins, uncovered, until the glaze is bubbling and just starting to char, and the lamb is really tender. Mix the yogurt, most of the pomegranate seeds and mint leaves in a small bowl. Shred the lamb at the table and serve with the sticky onions and the minty pomegranate yogurt. Scatter over the remaining pomegranate seeds.