In a small bowl add the saffron strands with 2 tbsps of hot water and a small knob of butter. Stir until the butter melts and the colour from the saffron has released and set aside.
Finely slice 2 onions and place them on some kitchen towel and sprinkle some salt to release their moisture and leave them for 20mins.
Meanwhile, using a pestle and mortar, pound the blade mace and cardamom seeds into a fine powder and set this aside also.
Squeeze all the excess moisture from the sliced onions and deep fry them in a small pan until they have become golden and drain them on kitchen towel and set aside.
Put half of the fried onions into a food processor with 4 tbsps of hot water and make a puree.
In a mixing bowl add the diced lamb and the grated ginger, sliced garlic, chilli powder and onion puree and mix altogether until all the meat is coated. And leave to marinate for 1 hour.
Dice the remaining 2 onions and put them in a pan with 6 tbsps of groundnut oil and place on a medium heat for 5 mins, until the onions have soften but not coloured. Then add the cinnamon stick, green cardamoms, black cardamoms, cloves and bay leaf and fry for 30 seconds, until you have a warm aroma.
Add the lamb, with all its marinade, and the garam masala, green chilli's and yogurt. Give it a good stir, cover and leave to simmer for 45 mins, turning occasionally.
Meanwhile, rinse the rice alternating from hot and cold water, until the water runs clear. Fill with cold water and leave to soak for 20 mins.
Preheat the oven to 160C/ 320F/ gas mark 3.
Drain the rice and add hot water and bring to the boil for only 5 mins, then drain, so the rice is only half-cooked.
In a oven proof pan, put half of the hot meat and cover with half of the rice. Add half of the finely sliced coriander, the pounded blade mace and cardamom seeds and half of the leftover fried onions.
Repeat the process, by putting the rest of the meat mixture and cover that with the rest of the rice. Sprinkle the rest of the pounded blade mace, coriander and fried onions. Then spoon over the saffron liquid in random swirls and add small knobs of butter all around the top. Place a cut out greaseproof circle on top and cover with the lid. Bake for 45mins.