Mix the onions, garlic, ginger, turmeric, chili powder, cumin, coriander, slat, flour and egg together. Leave to marinade the flavours and rest the batter in the fridge for 30 minutes. Meanwhile make the mango chutney.
Fry the red onion in the oil for 5 minutes or until softened. Stir in the mustard seeds and red chili and cook out for 1 minute. Add the chopped mango, lime juice and sugar and stir through. Cook on a low heat for 15 minutes or until the mango has cooked through. Remove from the heat and leave to cool. Stir though the mint then its ready to serve. You can make this a couple of days in advanced if you like and it does seem to get better with age.
Heat the oil in a large saucepan. Take a tablespoon of the batter and squeeze out any excess liquid, the drop into the oil and fry all over for 2-3 minutes, turning halfway through. Only do 4 – 5 tablespoons at a time so that they can brown nicely. Repeat with all of the mixture
Serve the onion bhajis hot with the quick mango chutney.