Fry the onion in oil till brown. Remove the onion and grind it with chopped tomatoes and yogurt and keep it aside. 

In the same oil add turmeric,coriander powder, red chillie powder, garlic, ginger paste and salt. 

Fry with little water till water dries. 

Add mutton and cook till water dries again. Add the onion tomato paste and fry well till the oil comes up. 

Add 3 to 4 glasses of water, cover and cook till the meat tenders. 

When meat is almost done add potatoes and cook till the potatoes are cooked and the your required gravy is left. 

Add green chillies and cook throughout on low heat. 

Serve with naan/chapati/boiled rice.