Thoroughly wash the mutton pieces and drain all the water out. Take them in a large bowl and start the marinating process.
For marinating the mutton, add ginger paste, garlic paste, curd/yogurt, salt, turmeric powder, Kashmiri red chilli powder, coriander powder, cumin powder and red hot chilli powder, if using. Also add a couple of teaspoons of mustard oil.
Now start mixing the mutton with all the spices and curd, preferably using your hand to coat each piece of meat evenly on all sides. Take some time at this step to give the meat a good massage.
Cover the bowl with a plastic wrap and let it marinate in the refrigerator for at least 4 hours or preferably overnight.
When you are ready to cook, heat the mustard oil in a large pressure cooker. You may also slow cook it in a large heavy bottomed stock pot; but I preferred cooking it in a pressure cooker to reduce the cooking time.
Once the oil is hot, temper it with bay leaf, cloves, black & green cardamoms, cinnamon stick and dried red chilies. Also add the sugar. Let the whole spices crackle and become fragrant for a minute or so on low heat ensuring they do not burn.
Now add the sliced onion and start sautéing them in medium heat. Add the sliced green chillies and keep sautéing the onion until it becomes deep golden brown. This would take a good 15 minutes on low medium heat.
Once the onion is nicely sautéed and has become soft and golden brown, add the marinated mutton pieces. Using a firm stirring spatula, stir the mutton to mix them well with onion and oil.
Now you have to keep sautéing the mutton on low heat stirring occasionally to ensure the spices do not stick to the bottom of the pan
Mutton will release a lot of water, so do not add any water at this stage. You need to sauté the mutton until all the water is dried up and oil starts to release; this will take a good 45 minutes to 1 hour. Do not rush this step.
Once oil starts to release from mutton pieces and spices keep sticking to the bottom of the pan, it’s time to add water. Add about 1 to 2 cups of hot water and give a good stir to mix the water with spices and meat.
Let the gravy come to a boil. Then tightly close the cooker lid with the whistle on. Let the mutton kosha cook on high until the first whistle blows.
After the very first whistle, reduce the flame to lowest and leave it for another 5 minutes; switch off the flame after exactly 5 minutes and allow the cooker to release the pressure on its own.
Once the pressure is completely released from the cooker, unlock the lid carefully and check if the mutton is completely cooked through. Try poking a piece of meat with the edge of a spoon and if the meat falls apart easily, mutton is fully cooked.
If the mutton pieces are not cooked through yet (which may happen due to the quality of the meat), put the lid back on and cook it on medium flame until another whistle blows. Allow the pressure release on its own and then check the doneness of the meat again.
Mutton kosha ideally should not have too much gravy, it would have oily spices clinging to the meat. If the gravy looks too thin, boil off the excess water by putting the cooker on high heat for 5 minutes.
Check and adjust the seasoning if needed by adding salt as per your taste. Finally, add garam masala and drizzle ghee on top. Give a final mix and serve the mutton kosha with plain hot Basmati rice. Enjoy!
Recipe by Flavor Quotient at https://www.flavorquotient.com/2024/03/mutton-kosha-classic-bengali-kosha-mangsho-recipe/