In an electric mixer, blitz the fish, coriander leaves and a pinch of salt to a smooth paste. Keep in a bowl. To the paste, add the fried onions, chopped ginger, green chillies, garam masala powder, turmeric powder and roasted cumin powder. Add in the egg yolk. Mix well.
Make small round koftas of the fish paste. Keep aside in a refrigerator for 30 minutes or so.
Heat oil in a pan and deep fry the koftas till golden brown. Remove the koftas from the oil using a slotted spoon, drain the excess oil using a kitchen towel and keep aside.
To make the gravy for the koftas, heat oil in a frying pan. Add the whole cumin seeds and allow them to splutter.
Add the onion paste, garlic paste, ginger paste and green chilli paste. Saute for a couple of minutes. Add in the coriander powder, turmeric powder and salt, stir well. Cook over a medium flame till oil starts to separate from the masala.
Now gently fold in the whipped yogurt and cook for another 3-4 minutes. Add 1/2 a cup of warm water and bring to a simmer.
Gently place the fried koftas in the gravy and cook over a low flame for a further 5-7 minutes.
Finish with ghee and garam masala powder. Serve hot.