1. Combine the yoghurt,garlic,ginger,garam masala,turmeric,chilli and lamb pieces and refrigerate for at least 4 hours.
2. Heat oil in a large casserole dish over medium heat and fry the cashews until golden brown.
3. Remove cashews and set aside.
4. Fry the onions until golden brown. Remove onions and set aside.
5. Add the lamb to the oil and fry until browned,then return the onions,reserving 1/2 a cup for garnish,and add cinnamon quill,salt and 3 cups of water.
6. Cover and simmer for 1 1/2 hours until the lamb is tender,stirring occasionally.
7. Wash the rice and quinoa and cover with 5 cups of water in a microwave-safe bowl.
8. Microwave on high for 7 minutes,then remove and drain.
9. Stir the potatoes and the par-cooked rice and quinoa through the lamb and add the milk,saffron,sultanas and butter.
10. Cover tightly and cook on very low heat for 20 minutes.
11. Remove from heat and allow to stand for 10 minutes before uncovering.
12. Scatter with fried onion,cashews and coriander leaves and serve with the coriander raita.
For a family feast serve this biryani with Adam'svegetable curry puffs (cheat's samosas).