Marinate mutton pieces with whisked yoghurt, salt, turmeric powder, 1 tsp Kashmiri red chilli powder, 1 tsp red chilli powder, ginger paste, garlic paste and 2 tbsp mustard oil. Keep this overnight.
Take half the quantity of onions and make a paste. Cut the other half of the quantity into thin slices.
Take all the powdered spices except garam masala and mix them all together with a little bit of lukewarm water to make a thick paste.
In a heavy-bottomed kadai or degchi (saucepan), heat mustard oil and add the whole spices and sugar. Let the sugar caramelize and then add onion slices.
Fry the onions on high heat and when they have become soft, add the onion paste. Keep frying the onion paste till the raw smell of the paste goes off.
At this point, add the mutton after shaking off the marinade.
On high heat, fry the mutton pieces, stirring throughout to brown them.
Once that is done, add the tomato puree, the spices paste and the leftover marinade.
Use a little warm water to help all the spices and the mutton come together.
Mix them all up and keep frying the mutton with the spices for a good ten to fifteen minutes.
Reduce the heat and cover the mutton and let it cook for 30 minutes, while stirring time and again so that the spices don't stick to the bottom.
If you wish, you can continue cooking this way, adding 2 cups of hot water mid-way and then covering and letting the mutton cook. Or alternatively, you can transfer the mutton to a pressure cooker, add water to cover the mutton pieces and then close the lid and cook for 5 to 6 whistles or until the mutton has become soft.
Once the mutton has cooked all the way, add Bengali garam masala, check the salt and add more salt if needed and then finally cut the green chillies into half length-wise and add to the mutton.
Give it a boil and then serve this with piping hot rice.