Prepare the marinade by pounding all the ingredients together in a mortar and pestle. It has to be a coarse paste.
Clean, wash and pat dry the fish.
Season the fish with salt and then apply the marinade. Keep for at least 6 hours, preferably overnight.
In a pan, heat oil, crackle mustard seeds and add the cumin seeds. Slide the fish in and cook for 5-6 minutes on one side.
Gently turn over and add the curry leaves on top. Cook covered for another 5-6 minutes or till the fish is done.
Serve hot with lemon wedges.