Season the mackerel with salt and pepper and let marinate for 15 minutes.
Pound the garlic, shallots and red chilli in a mortar and pestle until fine.
Heat oil in pot and fry the pounded ingredients and curry leaves until fragrant. Then add the fish and fry together. Mix the curry powder with some water to make a paste and stir in the pot. Add the tamarind paste, water, milk and mix.
Add the tomatoes, eggplant and onion then season with sugar, salt and pepper. Bring to a boil, lower heat, taste and adjust seasoning. Simmer for another 5 minutes then serve with rice.
I use the concentrated seedless tamarind paste which can easily be found in stores nowadays. If you are using the tamarind blocks, then pinch off a larger section and soak in hot water then strain the seeds to get the juice.