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For the marinade, blend or pound the ginger, garlic, lemon juice, cumin, chilli powder, 1⁄2 tsp ground black pepper, 11⁄2 tbsp water and 2 tsp salt to a fine paste, then stir in the ajwain seeds. Place the fish in a non-metallic bowl, spoon over the marinade and toss well to coat. Leave to marinate in the fridge for 1 hr.
Mix the rice flour and gram flour together. Sprinkle over the fish, thoroughly coating it, then set aside for 10 mins.
Fill a wok or large saucepan with oil so that it comes between half and two-thirds of the way up the side, and heat to 190C – you could use a deep-fat fryer for this. Drop a few pieces of fish into the hot oil, but do not overcrowd the pan, and fry for 2-3 mins until golden brown. Transfer to a plate lined with kitchen paper to drain. Repeat with the remaining fish. Sprinkle over the chaat masala and serve immediately.