Soak rice for half an hour in five to six cups of water. Drain and keep aside. Cut half the ginger into julienne and make a paste of the rest. Soak saffron in warm milk and keep aside.
Marinate mutton in yogurt, salt, turmeric powder and one tablespoon each of ginger and garlic pastes. Marinate for about four hours in a refrigerator. Heat sufficient oil in a kadai and fry half of the sliced onions till brown and crisp.
Drain and keep onto an absorbent paper. Cook rice in salted five cups of boiling water along with cloves, cinnamon, green and black cardamoms and peppercorns till rice is almost cooked.
Strain and keep rice warm. Heat ghee in a thick-bottomed pan or pressure cooker. Add remaining sliced onions and green chillies. Cook, stirring continuously, till onions are light golden brown. Add remaining ginger paste and garlic paste and mix well.
Add marinated mutton and cook on high heat for seven to eight minutes. Add coriander powder, cumin powder and red chilli powder. Mix thoroughly.
Stir in three cups of water, bring it to a boil, reduce heat and cook covered till mutton is almost cooked.
Add tomatoes, salt, half the garam masala powder and fresh coriander leaves. Cook for fifteen minutes on medium heat, stirring occasionally.
In case you are cooking the mutton in a pressure cooker, add chopped tomatoes, salt, garam masala powder, two cups of water and chopped fresh coriander leaves after adding dry spices.
Pressure cook till mutton is almost cooked. It normally takes two to three whistles to cook. Ensure that the cooked mutton does not have a thin gravy. If that is the case cook on high heat to reduce.
Preheat oven to 180 degrees. Arrange half the quantity of cooked mutton in an ovenproof dish and spread half the quantity of cooked rice on top of the mutton. Sprinkle a little of the remaining garam masala powder, half each of ginger julienne, saffron dissolved in warm milk and mint leaves.
Dot the rice with half the quantity of butter. Spread the remaining mutton on top of the rice, followed by cooked rice and repeat the earlier process with the remaining quantities of ginger julienne, saffron milk, garam masala powder, mint leaves and butter.
Cover it with aluminium foil and cook in the preheated oven for 15 to 20 minutes at 180 degrees. Garnish with fried sliced onions and serve hot