Clean and pat dry the seer fish pieces. Season with salt.
Grind all the ingredients under list 3 to make a paste like marinade (add water if necessary).
Marinate the fish pieces using this paste, coating both the sides well. Keep for at least 4 hours.
Heat the ghee in a large, flat pan and shallow fry the seer fish pieces in batches. Keep aside.
In another pan, heat the mustard oil to smoking hot and then add the onions; sauté for two minutes.
Lower heat and add the coriander seeds, cardamom pods and cumin seeds. Saute for another 2-3 minutes and then add the turmeric powder, garam masala, kasuri methi and nutmeg.
Continue to sauté till the onions are light brown and mixed well with the spices. Next, add the tomatoes and cook till the sauce thickens and becomes a mushy consistency. You can also notice oil separating at the sides of the pan.
Season with salt and chilli powder, if necessary.
Place the fried fish carefully on the sauce and spoon some over the fish pieces.
Garnish with coriander leaves, green chillies and ginger just before serving.