1. Make a few deep cuts in the surface of the lamb. Place the garlic cloves and ginger with the lemon juice in a food processor or mini chopper and blend to form a paste.
2. Place the saffron strands (or alternatively use turmeric) in a small bowl with the sugar and salt and pour over 1tbsp hot water. Let it soak for 5 minutes. Mix together the garlic paste, saffron, chilli powder, yogurt and 2 tbsp oil. Pour over the lamb and rub into the cuts. Marinate for 4-6 hours or preferably overnight.
3. Preheat the oven to 170°C, gas mark 3.
4. Heat the remaining oil in a large casserole dish which is large enough to fit the lamb. Add the sliced onions and whole spices and sauté until softened but not brown.
5. Add the leg of lamb then either seal with foil or a lid or both. Place in the oven and cook for 1½ hours. Remove the foil and lid and baste. Cook for a further 30 minutes without the foil or lid until the lamb is golden brown, really tender and completely cooked through.
6. Allow the lamb to rest for 10 minutes before carving. Spoon over the onions and pan juices to serve.
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