Heat your oven to 220°C/Fan 200°C/Gas 7. Unwrap the lamb and put it in a snug roasting tin. Pat the meat dry with kitchen paper. Set aside.
Tip 1 tbsp coriander seeds into a dry frying pan and toast over a medium heat for 2-3 mins till aromatic and popping. Tip into a pestle and mortar and crush as finely as you can. No pestle and mortar? Use a small bowl and the end of a rolling pin or a jam jar.
Stir 1 tbsp cumin, 1 tsp cinnamon and a pinch of chilli powder (it’s spicy, so use as much or as little as you prefer) into the crushed coriander. Add a pinch of salt.
Peel and crush or grate the garlic cloves and add them to the spices. Grate in the lemon zest and squeeze in the juice from 1 lemon half. Add 1 tbsp olive oil and stir to make a paste. Rub it into the lamb, making sure you cover it all over. Leave for 30 mins to marinate if you have time.
Roast the lamb for 25 mins, or till it has a golden crust. Take out of the tin, cover the tin with a double layer of foil and return the tin to the oven. Lower the heat to 160°C/Fan 140°C/Gas 3 and roast the lamb for 3 hrs.
Remove the tin from the oven. Carefully unwrap the lamb (be careful to keep your fingers safe from any steam that escapes). Transfer the meat to a board and loosely cover with foil. Rest for 45 mins to 1 hr before serving with the juices from the tin spooned over the top.