I prefer Iron pan to make the Kosha Mangsho as slow cooking over Iron wok gives the blackish color to the mutton.
You can use a pressure cooker to make the Kosha Mangsho as well to avoid the extra time required for cooking. However, cooking over Iron pan, given, the desired color to Kasha Mansho.
Keywords: Kosha mangsho recipe, mutton kosha recipe, debjani rannaghar
Find it online: https://kitchenofdebjani.com/2014/11/kosha-mangsho/