Soak the onion in cold water for at least 5 minutes. In a large bowl, sift together the dry ingredients then add the chopped chilli and coriander. Add the cold water to the flour mix to make a batter. Adjust with more water if necessary.
Remove the onion from the cold water and ensure it is drained well before transferring to the bhaji batter. Mix until well combined.
Heat an inch of oil in a large deep bottomed pan. Use your hands to shape the bhaji batter into round balls. When the oil is hot enough lower spoonfuls of the batter mix into the hot oil. Depending on the size of the pan you can do a few at a time but don’t overcrowd them. Cook for a minute or so on each side, turning until they are evenly browned and crisp. Remove with a slotted spoon and allow to drain on a paper towel and sprinkle extra salt if desired.
To make the raita, place the cucumber over a colander and squeeze the excess moisture out. Transfer to a bowl and add the yoghurt and mint and stir well.
Serve immediately with raita and garnish with extra coriander.