To make the garam masala, blitz all the ingredients in a coffee grinder/mixer till finely powdered.
Marinade the mutton for at least 6-8 hours or overnight in yoghurt, ½ tbsp mustard oil, salt, ½tsp each of chilli powder, turmeric and coriander.
Heat 1 tbsp mustard oil in a heavy-bottomed, deep pan or Dutch oven and fry the potatoes (sprinkling salt lightly) till golden.
Add the remaining mustard oil to the hot pan and add all the ingredients for the paanch phoron (tempering). Allow them to pop and crackle but see that they don't burn.
Add the ground onions and sugar and saute on medium heat for 15 minutes or till lightly golden. Tip in the chopped onions and cook for a further 10 minutes till you have a golden brown mixture.
Now add the turmeric, garlic and ginger pastes and cook on a low flame ensuring that they don't burn.
Next add the chilli powder and 1 tsp garam masala, stir for 20 seconds and put in the marinated meat. Raise the flame to medium-high and lightly sear the meat for 1-2 minutes.
Now pour in about 250 ml water, bring the curry to a boil, lower the heat, cover and simmer the curry for 20 minutes, checking every 10 minutes and adding a little water (only if needed). The meat will release its juices too.
After 20 minutes, the meat should be almost done. Now toss in the fried potatoes, check the seasoning, cover and cook for a further 20 minutes.
When the meat is falling off the bone and the potatoes are cooked through, add 1 tsp garam masala and correct the seasoning one final time before turning off the heat.
Heat the ghee in a small pan, turning off the heat before it smokes. Add the cinnamon powder and pour it over the mutton curry. Mix it in and serve.