Soak the onion in cold water while you make the base mix. Sift the flour and baking powder into a bowl, then add the chilli powder, turmeric, chopped chilli and a good sprinkling of salt. Mix in about 100ml of cold water to make a thick batter – add a splash more if it feels too stiff.
For the raita, peel the cucumber and grate it into a sieve set over another bowl. Mix the remaining ingredients with some seasoning and the drained cucumber – squeezing out any extra moisture with your hands – then spoon into a small serving bowl.
Drain the onion well and mix it into the batter. Heat about 5cm of oil in a wok or deep pan. Do not fill the pan more than a third full. Add a tiny speck of batter. If it rises to the surface surrounded by bubbles and starts to brown, then the oil is hot enough for frying.
Lower heaped tbsps of the bhaji mixture into the pan, a few at a time, and cook for a few mins, turning once, until they are evenly browned and crisp, so about 3-4 mins. Drain on kitchen paper, sprinkle with a little salt and keep warm while you cook the rest. Serve with the raita.