Heat oil in a pan & add all the ingredients listed from coconut - red chilli (listed under for masala). When the coconut becomes golden brown remove from fire. Let it cool completely.Grind the cooled coconut mixture, without water & keep aside.
Heat oil in a pressure cooker. Saute onion, ginger & garlic. When onion becomes soft add all masala powders & stir for 5 minutes. Add curry leaves. Add medium thick coconut milk.
When it starts boiling add mutton pieces. Cover the cooker with the lid and put the weight on. After the first whistle on full flame, reduce the flame to low-medium and cook for 12-14 whistles. Keep the cooker closed for another 10-15 mins, till the pressure drops.
Open the cooker and mix well. Check the doneness of the mutton, if required, pressure cook again. Add 2-3 tbsp hot water to the ground coconut mixture and add this to the cooked mutton. Mix well.
Add thick coconut milk & keep for 5 -7 mins. Switch off the gas.
Heat oil in a small pan & crackle mustard seeds. Add small onion & curry leaves. When onion becomes brown pour it over mutton curry.
Serve hot with Pathiri, chappathi or rice.
Notes
The cooking time of mutton depends on the type of mutton you use. The curry keeps well in the fridge for 2-3 days.