Preheat the oven to 180C/350F/Gas 4.
For the aubergine fritters, heat the oil in a pan and gently fry the onion and garlic for 2-3 minutes. Add the turmeric, tomato purée, stock, wine and sugar and bring to a gentle simmer for about five minutes.
Meanwhile, heat the vegetable oil for deep frying in a large saucepan and heat until a small cube of bread turns golden-brown in 30 seconds. (CAUTION: do not leave oil unattended as hot oil can be dangerous.) Carefully add the aubergine cubes and fry for 4-5 minutes, or until golden-brown.
Remove with a slotted spoon and drain on kitchen towel before stirring them gently into the onion and spices. Finally, stir in the coriander.
For the cumin and turmeric crusted mackerel, combine the spices in a bowl and coat the fish with the mixture. Season, to taste, with salt and freshly ground black pepper. Heat the oil in a frying pan and fry the mackerel until golden-brown on both sides, about 3-4 minutes. Transfer the pan to the oven to continue cooking for 8-10 minutes.
To serve, place the fish in the centre of a serving plate and surround with the aubergine fritters. Garnish with chopped fresh chives and lime wedges.