For the chutney
Lightly toast the coriander seeds, cumin seeds and black onion seeds in a hot, dry pan for 30 seconds.
Put the vinegar and sugar in a small pan and heat until the sugar has dissolved then add the toasted seeds, the salt and chile and let bubble for 5 minutes then add the mango and cook very gently for 20 minutes, stirring from time to time, or until very thick, then set aside.
For the bhajis
Lightly toast the coriander seeds and cumin seeds in a hot, dry pan.
Sift the flour into a bowl and add the toasted seeds, baking powder, salt and grated onions.
Add just enough water to make a very thick batter.
Heat the oil in a deep pan until bubbles appear on a wooden spoon held in the oil.
Drop small balls of the batter into the oil in batches and deep fry for 3 - 4 minutes or until golden brown.
Drain on kitchen paper and serve with the mango chutney.