Slice onions finely and slit green chilies. Set aside. Take the mutton liver in a wide bowl and add in turmeric powder and vinegar/lemon juice or milk. Soak them together for 20 minutes and after that wash it 4 to to 5 time in water. Soaking helps to get rid of the strong smell from the liver.
After washing the liver thoroughly, cut them into bite sized pieces and keep it ready along with other ingredients. Heat a pressure cooker with oil. Add in cumin seeds & let it splutter.
Then add in onions, curry leaves, green chilies along with ginger garlic paste. Saute them nicely until golden brown and the raw smell of ginger garlic paste leaves. Now add red chilli powder, coriander powder and required salt.Fry everything in low medium flame for 3 minutes.
Add cleaned liver pieces and fry until the color is slightly changed.
At this stage add in freshly ground pepper powder and give a quick stir. Then add ½ cup of water and bring it to a boil. Then close the lid of pressure cooker and cook everything in a low medium flame for two whistles or the mutton liver gets cooked well. You can also cover with a lid and cook until the liver is soft instead of pressure cooking.
Once the pressure releases, open the cooker and transfer the gravy to a thick bottomed pan.
Stir fry everything in medium low flame until the curry gets semi dry or completely dry. Consistency is purely your preference. Do not overcook as the liver will get hard. Switch off the flame once done and garnish with coriander leaves if preferred.