In a large bowl, combine the chickpea flour, cumin powder, coriander powder, chili powder, turmeric powder, and salt. Gradually add the water and whisk the mixture until it forms a smooth, thick batter.
Add the sliced onions to the batter and gently toss to ensure each slice is evenly coated.
Heat 2-3 inches of vegetable oil in a deep-frying pan or wok to 350°F (175°C) to 375°F (190°C).
Carefully drop spoonfuls of the battered onion mixture into the hot oil, making sure not to overcrowd the pan.
Fry the onion bhajis for 2-3 minutes, or until they are golden brown and crispy, flipping them occasionally.
Using a slotted spoon, transfer the fried onion bhajis to a paper towel-lined plate to drain excess oil.
Serve the onion bhajis hot, garnished with fresh coriander leaves (cilantro) if desired.