Lightly salt the haddock and leave to rest for 30 minutes.
To make the batter, put the cornflour in a bowl and add the remaining ingredients to make a thick batter.
Heat the deep-frying oil to 180C in a deep-fat fryer or a deep, heavy-based saucepan. (CAUTION: hot oil can be dangerous. Do not leave unattended.) Dip the haddock peices in the batter, letting the excess drip off, and deep-fry for about 3 minutes, until crisp and golden brown. Transfer to kitchen paper to drain and keep hot.
Heat the oil in a frying pan. Add the curry leaves, mustard seeds, garlic, onion, chilies, broccoli and peppers and stir-fry over a high heat for 2 minutes. Add the fried haddock, the ginger, salt and sugar and continue stir-frying for 2 minutes. Squeeze in the lemon juice, whisked yoghurt (if using) and sprinkle with the fresh coriander.
To serve, put the yoghurt and turmeric in a bowl and mix well. Heat the oil in a small frying pan and add the seeds. Once they start popping, tip them onto the yoghurt. Serve the gobhi kempu and haddock with the yoghurt.