Remove the head and tail from the mackerel, then cut each fish into 3 portions.
Rub the fish with the turmeric and half a teaspoon of salt, and set aside.
Cut the shallot in half lengthways, then peel it and cut it into 2mm (⅛") thick half-moon shapes.
Peel the garlic cloves and add them to a mini blender.
Peel the ginger and add it to the mini blender.
Add the cumin seeds, coriander seeds, cloves, dried red chillies, black peppercorns, blade of mace, dried pomegranate seeds and vinegar to the mini blender and blitz to a smooth paste. You can use a pestle and mortar for this too.
Heat the oil in a wok over a high heat and add the slit green chilli, curry leaves and slit green chilli and cook for 30 seconds.
Add the tomatoes and spice paste and cook for a minute.
Throw in the tomato paste and cook for a minute, stirring continuously.
Pour in the water, season with the remaining salt and add the jaggery and cook for 5 minutes.
Add the fish and cook for 3 minutes, then flip the fish and cook for another 2 minutes.