Heat oil in a pressure cooker and fry the sliced onion till soft.
Add 1 tbsp ginger & garlic paste, 1 tbsp red chili powder and 1 tbsp coriander powder.
Fry well on low heat for 1 minute. Then add the lamb shanks and fry well with the spices till well coated for about 2 to 3 minutes.
Now add 5 to 6 cups of water and remaining pieces of bones. Add salt and 2 tbsp prepared nihari masala. Stir well and cook on medium heat till meat is well cooked. (in a pressure cooker)
Once the meat is cooked, release the pressure in the cooker and open the lid.
Continue cooking the gravy on low heat.
Make a mixture by adding 4 tbsp of wheat flour with 1 cup of water. Whisk well so that there are no lumps formed.
Start pouring this mixture into the gravy little by little, and keep stirring the gravy to avoid any lumps.
Finally, add sugar and lemon juice and also taste and check the salt.
Let the nihari start to boil, reduce the heat and cook for 5 minutes.
Garnish with fried onion, sliced ginger, and green chilies.
Serve hot with naan or roti.