In a large bowl, mix all the ingredients for the marinade. Refrigerate the bowl for 3-4 hours and marinate the mutton well.
Heat mustard oil in a pan. An iron pan is best to make this dish as it gives a deep blackish color to the dish but if you don’t have one, use any heavy bottom pan.
Slightly crush black cardamom, green cardamom, cinnamon, and cloves. Crushing the spices helps in releasing all their flavor in the oil. When the oil is hot, add bay leaf, crushed spices, dry red chilies, and sugar and fry for a few seconds.
Add thinly sliced onion and fry on medium high heat until slightly browned. Keep stirring at regular intervals.
Add ginger and garlic and fry until the onions are nicely browned.
Now add tomatoes and cook for 2 minutes.
Add the mutton pieces and potatoes along with the marinade and fry for 5-6 minutes on high heat.
Add the dry spices and salt and cook for 2-3 minutes. Now add a cup of water and cover the pan. Cook for 1.5 to 2 hours until the mutton is tender. Keep stirring in between and scraping the sides and bottom of the pan. Add more water while cooking if the mutton is getting dry. The consistency of Kosha Mangsho is slightly thick, so keep that in mind.
Finally, add garam masala powder and lime juice and mix well.
Cook for another 3-4 minutes until the gravy is slightly thickened.
Garnish with fresh coriander and serve hot.
Some people also add raw papaya as a tenderizer but I have not added it. If you wish, you can add 2 tsp of grated raw papaya while marinating the meat.If you are short on time, cook the curry in a pressure cooker. Once the mutton is added, close the lid of teh cooker and pressure cook for one whistle on high heat. Then simmer the heat to low and cook for 20 minutes. Let the pressure release naturally and then open the lid.This dish tastes best the next day. So if you have planned it before, make it a day prior to serving.To make a restaurant-style smooth gravy, you can also grind the fried onion masala and then add it back to the pan.Potatoes are a must-add in this curry and no Kosha Mangsho is complete without adding a few.You can drizzle a few teaspoon of ghee over the ready gravy just before serving.