Kosha mangsho is oh so good to know. It is not a quick recipe but who cares. So worth it.
Marinate mutton in yogurt, a little mustard oil, and salt and turmeric, overnight.
Roast whole garam masala (whole cumin, coriander, nutmeg, cinnamon, cardamom, cloves, and black pepper) on a dry pan for a bit, and grind into a powder. Keep aside for cooking. Fresh garam masala adds a whole new kick.
Get yourself a nice thick bottomed pan for which you have a lid ready. Heat some mustard oil, and fry chopped onions. Add salt and a pinch of sugar, and a little of the garam masala you made. Saute together till it all starts to melt together. Add the mutton. Add some more coriander and cumin powder if you want, and some red chilli powder. Add some chopped tomatoes and then some more of that garam masala (leave a little to sprinkle in the end and add the rest, basically). Cover with the lid.
Now comes the painful part of standing there and constantly stirring the pot. You have to keep lifting the lid and stirring so the bottom doesn't burn, and then replacing the lid. (If it gets too dry add very little water just to keep it a little mushy, but that's it.) This process is called koshano in bengali. It would be so much easier to just pressure-cook the mutton but DON'T DO IT! Stir, cover, stir, cover, till the mutton is cooked. I took it out right after it was cooked because I was impatient, but typically you want it to almost start disintegrating before you turn off the heat. Sprinkle the leftover garam masala on top and you're set! Serve with luchi and be amazing.
PS: It's also common to put potatoes in this but I like to leave them out.