Trim off the parchment-like skin and surface fat from the lamb, or get your butcher to do it. Slash it all over and place in an oiled roasting tray. For the first marinade, in a frying pan, toast the poppy seeds until they exude a nutty aroma and become a few shades darker. Put into a bowl with the almonds, raisins and cashews.
Pour over 80-100ml of boiling water and soak for 15 minutes. In the same pan, dry-fry the onion chunks until they soften and form brown patches. Add to the nut mixture. Put the nut and onion mixture, with the soaking liquid, into a blender with the garlic, ginger, chillies, coriander, lemon juice and salt. Blend to a fine paste. If the paste is too coarse, add a little water. Massage the mixture over the lamb and into the slashes. Cover and refrigerate for two to three hours.
Heat 3 tbsp of the ghee in a frying pan over a medium heat and add the sliced onions. Fry until golden brown. Remove with a slotted spoon and drain on kitchen paper, reserving the ghee. Put two-thirds of the onions into a blender with the yogurt and saffron milk. Blend to a fine paste. Put all the spices for the spice mix into a frying pan and gently toast until they release their wonderful aromas. Remove from the heat and grind to a fine powder in a coffee or spice grinder. Mix 2-3 tsp of the spices into the yogurt mixture and season with salt (store the rest in an airtight container for up to six weeks; you’ll definitely use it again!). Allow to infuse for an hour. Remove the lamb from the fridge and massage in the yogurt mixture. Return to the fridge and marinate overnight.
Preheat the oven to 200°C/400°F/gas mark 6. Return the lamb to room temperature. Pour over the ghee from the onions, the remaining 2 tbsp ghee and 300ml water. Cover with foil and cook for 30 minutes, basting once. Reduce the oven temperature to 170°C/340°F/gas mark 3 and cook for about three hours, basting every 30 minutes, until the meat falls off the bone, adding a further 300ml water halfway through. If the sauce starts to dry out too fast, keep adding water and continue to baste.
Serve on a platter garnished with toasted flaked almonds, the reserved fried onions and chopped mint.
Recipe courtesy of Reza’s Indian Spice by Reza Mahammad, published by Quadrille (£17.99, hardback). Photos © Martin Poole.