Place the flour into a large bowl. Gradually add the water, whisking constantly, until a thick batter is formed. Add the chilli and mix in well.
Heat the two tablespoons of vegetable oil in a frying pan over a medium heat. Add the onion and fry until soft, then remove with a slotted spoon.
Place the prawns, salmon and haddock into a clean bowl and add cooked onion, stirring well to combine.
Heat the 400ml/14fl oz of vegetable oil in a deep, heavy-bottomed saucepan until a breadcrumb sizzles and turns brown when dropped in it. (CAUTION: hot oil can be dangerous - do not leave unattended!).
Using a tablespoon, take even amounts of the fish mixture and dip into the batter mixture, then carefully place into the hot oil. Deep-fry for 8-10 minutes, or until crisp and golden and completely cooked through, then carefully remove with a slotted spoon and drain on kitchen paper.
To serve, place the bhajis into a serving bowl with the coriander, mango chutney and yoghurt in bowls alongside for dipping.