In a large bowl, mix ½ kg bone-in goat mutton, ½ cup thick yogurt, 1 tsp ginger paste, 1 tsp garlic paste, and 1 tbsp mustard oil. Refrigerate the bowl for 3-4 hours and marinate the mutton well.
Heat 4 tbsp mustard oil in a large 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. I used my steel kadai.
Slightly crush 2 whole black cardamoms, 3-4 whole green cardamoms, 1-inch piece of whole cinnamon stick, and 2-3 whole cloves in a mortar and pestle. Crushing the spices helps in releasing all their flavor in the oil. When the oil is hot, add 2 whole bay leaves, crushed spices, 3-4 whole dry red chilies, and 1 tsp sugar to the pan and fry for 4-5 seconds.
Add 1 and ½ cups of thinly sliced onion and fry on medium-high heat until they turn slightly brown. Keep stirring at regular intervals. It will take 6-8 minutes.
Add 1 tsp chopped ginger and 1 tsp chopped garlic and fry until the onions are nicely browned. It will take 8-10 minutes.
Now add ½ cup chopped tomatoes and cook for 2 minutes.
Add the marinated mutton pieces along with the marinade and 2-3 medium size potatoes cut into halves and fry for 5-6 minutes on high heat.
Add the dry spices like 2 tsp coriander powder, 1 tsp turmeric powder, 2 tsp Kashmiri red chili powder, 1 tsp cumin powder, and salt to taste, 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. Add more water while cooking if the mutton is getting dry and the masala is burning at the bottom of the pan. The consistency of Kosha Mangsho is slightly thick, so keep that in mind.
Finally, add ½ tsp garam masala powder and 1 tbsp lime juice and mix well.
Cook for another 3-4 minutes until the gravy is slightly thickened. Serve hot.
Make sure the yogurt is not sour.To make thick yogurt, simply transfer it to a fine sieve and let it rest for an hour in the refrigerator over a bowl. All the excess water will drip down and the yogurt will be thick and creamy. You can use Greek yogurt too. It doesn’t require hanging.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 the 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 teaspoons of ghee over the ready gravy just before serving.