First make the relish by mixing all the ingredients together in a bowl, season well and chill until ready to serve.
Separate the sliced onions in a large bowl. Add the chilli powder, turmeric, cumin seeds, crushed coriander seeds and season with sea salt. Mix well. Add the chickpea flour a little at a time, stirring again to coat the onions. Now gradually sprinkle some cold water over this mixture, adding just enough water to make quite a sticky batter that coats the onions. Use your fingers to mix thoroughly.
Fill a deep, wide saucepan one-quarter full with vegetable oil and place over a high heat until it reaches 180°C (350°F) or a cube of bread sizzles and turns golden in 10–15 seconds.
Drop spoonfuls of the mixture into the oil. Fry in batches over medium–high heat for 1–2 minutes or until golden brown and crisp on the outside. Remove with a slotted spoon and drain on kitchen paper. Serve immediately with the relish.