In a bowl, add mutton, yoghurt, ginger paste, garlic paste, red chilli powder, turmeric powder and mustard oil. Mix well. Cover with a lid and keep it in the refrigerator. Marinate overnight or at least 8 hours.
In a pressure cooker, heat mustard oil over a high flame. Once the oil is hot, lower the flame to medium. Add bay leaf, dry red chilli, black peppercorns, cloves, cinnamon, green cardamom and black cardamom. Fry for a few seconds until they are fragrant.
Add onions and sugar. Cook for 7-8 minutes or until the onions are light brown in colour. The sugar helps in caramelizing the onions. Add ginger and garlic paste. Cook for 5 minutes.
Add marinated mutton, red chilli powder and coriander powder. Mix everything together. Cook for 2 minutes.
Then add salt and give it a quick mix. Lower the flame and cover the pressure cooker with a bowl that is filled with water ( more than 1 cup). Cook for 5-7 minutes.
Check if the mutton has released water. If so, then increase the flame to medium-high and cook for another 10-15 minutes while stirring frequently.
In a saucepan, bring 1/4th cup water to a boil and then add tea leaves. Strain and keep aside.
Once all the water has evaporated and oil has started releasing from the mutton, pour the tea water, 1 cup hot water and garam masala into the cooker. Mix well.
Pressure cook for 1-2 whistles or until the mutton has fully cooked.
Garnish kosha mangsho with coriander leaves. Serve hot with luchi (pooror steamed rice.