Preheat oven to 175°C. Drop the saffron threads into small bowl or cup, and allow to soak in boiling water for 10 min. Bring ¾ pint of water to the boil in a heavy, medium-sized saucepan. Pour in rice, add 1 tsp of the salt and cook briskly, stirring constantly, over a moderate heat for 10 min. Strain the rice in a colander and set aside.
Heat 3 tbsp of the ghee over a high heat in a large, heavy frying pan until hot. Add onions, cumin seeds and the rest of the salt. Fry mixture for about 3 – 4 min, stirring constantly, until the onions are soft and transparent. Add ground cumin and turmeric, stir for 1 min longer, then add the lamb. Fry the lamb for about 10 min, until all traces of pink disappear, turning and breaking up any lumps. Remove the pan from the heat and stir in fresh coriander.
Heat the remaining 3 tbsp of ghee in a large, heavy casserole with a tightly fitting lid. Stir in the fennel seeds and rice. Evenly coat with ghee and remove the casserole from the heat.
Ladle 2/3 of the rice mixture into a bowl and spread the remaining rice evenly over the bottom of the casserole. Pour in ½ the saffron and its soaking liquid and add ½ the lamb mixture.
Spread ½ reserved rice over lamb. Add the rest of meat and rest of the rice mixture. Dribble the remaining saffron and soaking liquid evenly over the top. Add chicken stock, pouring it down the side of the casserole, and bring to the boil over a high heat. Cover casserole with a sheet of foil, and crimping the edges to hold it firmly in place.
Set the lid securely on top of the foil and bake in the middle of the oven for about 25 min, until the rice is tender and has absorbed all the liquid in the casserole.
Serve the Biryani straight from the casserole, or serve in a heated bowl or dish.