Kosha Mangsho (Mutton Kosha, Mutton Kasha) is a rich, slow-cooked Bengali mutton curry where goat meat is braised in mustard oil, yogurt, caramelized onions, and warming whole spices until the oil separates and the gravy turns deep brown, glossy, and intensely flavorful.
Rinse mutton well with water and drain nicely. In a large bowl, mix the mutton pieces, plain yogurt, ginger-garlic paste, and mustard oil. Cover the bowl with a lid and refrigerate it for 3-4 hours.
Make The Curry
Pull the marinated mutton out from the refrigerator and keep it aside on the counter.
Heat mustard oil over medium-high heat in a large pan. An iron pan is best as it gives a deep blackish color to the dish but if you don’t have one, use any heavy bottom pan.
While the oil is heating, slightly crush black cardamoms, green cardamoms, cinnamon stick, and cloves in a mortar and pestle. Crushing the spices helps in releasing all their flavor in the oil.
When the oil is hot, add bay leaves, crushed whole spices, dry red chilies, and sugar to the pan and fry for 4-5 seconds.
Add onions and cook until they turn slightly brown (6-8 minutes). Stir at regular intervals.
Add ginger and garlic and cook until the onions are nicely browned, stirring frequently. It will take 8-10 minutes.
Note: If you like a creamier gravy, then blend the browned onions at this stage using an immersion blender or a mixer. Then follow the rest of the recipe as it is.
Now add the marinated mutton pieces along with the marinade and potato halves and cook for 5-6 minutes on high heat.
Add the dry spice powders that are coriander powder, turmeric powder, Kashmiri red chili powder, cumin powder, and salt, and cook for 2-3 minutes.
Add plain yogurt and cook for 5-6 minutes.
Reduce the heat to low. Add a splash of hot water (50-60 ml) to the pan and stir to combine.
Cover the pan with a tight-fitting lid and cook for 15-20 minutes.
Now repeat the process of adding some water and cooking the mutton until the meat is tender (1.5-2 hours). Scrape the bottom of the pan at regular intervals to incorporate the browned bits with the masala. This process will help the mutton to develop a rich brown color. Be careful to not burn the gravy at the bottom of the pan.
Check for salt and add more if needed.
Finally, add garam masala powder, slit green chilies, and ghee and mix well.
Cook for another 3-4 minutes until the gravy is slightly thickened. Serve hot.
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Notes
If goat meat is not available, you can use lamb in its place.