Heat mustard oil in a large pan (This dish is traditionally made in an iron pan but you can even use non-stick kadai). Wait for the oil to start releasing smoke and then add bay leaves, cinnamon, cumin seeds, chopped green chillies, cloves and green cardamom to it and saute well for roughly about 2-3 minutes.
Add thin sliced / chopped onions and cook on medium flame until the onions becomes translucent while stirring. You roughly need to stir for 5-8 minutes to make them translucent. Keep adjusting the flame if onions stick to the pan. It is always recommended to stir it often.
Now, add marinated mutton to the pan and toss well with onions. Cook for 8-10 minutes while stirring occasionally.
Include ginger garlic paste and sauté for a minute so that it mixes well with onions and mutton pieces.
Add all spices to the pan and mix well. Stir it occasionally and prevent the spices from burning.
Add in tomatoes and then salt, so that tomatoes release water quickly and turn mushy. Keep stirring on slow flame.
Add little water (half a cup) and keep stirring for 5-6 minutes so that all the spices get cooked well. Bear in mind to prevent the spices from burning.
Keep stirring time-to-time and incorporate even brown bits which would have stuck at the bottom of pan. We want to give the dish beautiful brownish rich color.
Once mutton is fork-tender and you are able to tear the pieces apart using your two fingers, it's time to garnish the masterpiece. Add 1 tbsp of ghee to the pan as it will give the dish richness and nice aroma. Now add chopped coriander to the pan.
Switch off the gas and transfer the scrumptious mutton kosa/kosha to a serving bowl. Garnish it with chopped coriander leaves. Serve the hot mutton kosa with salad and plain rice or parotta or pulao for your lunch or dinner. Follow it up with desserts (Rosagulla / Misti Doi).