Peel and halve the onions and then slice very thinly. Finely chop the garlic. Mix the arrowroot and chickpea flour and enough water with the remaining ingredients until creamy batter. Add the onions and the garlic. Heat the oil of 1 inch height in a large pan and add tablespoonfuls of mixture in batches. Once cooked put on absorbent towels. Serve with mango chutney.