Thinly slice the onions to about 3mm thickness, aiming for even strips that will cook uniformly and get crispy.
In a mixing bowl, combine the chickpea flour, cumin, coriander, chili powder, and salt, whisking together to evenly distribute the spices.
Add the sliced onions to the bowl. Pour in 2 to 3 tablespoons of cold water and use your hands or a spoon to mix everything thoroughly, creating a sticky batter that coats the onions well.
Heat the vegetable oil in a deep pan over medium-high heat until it shimmers and reaches about 180°C (350°F). You can test by dropping a small spoonful of batter—the sizzling and quick rise indicates readiness.
Using a slotted spoon, scoop about 2 tablespoons of batter and gently slide it into the hot oil, taking care to avoid splashes. Fry in small batches to prevent overcrowding.
Fry the bhajis for about 3-4 minutes per batch, turning occasionally with tongs, until they are deep golden brown and crispy on all sides. You'll hear a gentle crackling as they fry.
Once cooked, lift the bhajis out with a slotted spoon and drain on paper towels to remove excess oil. Repeat with remaining batter.
Serve the hot, crispy onion bhajis immediately, garnished with a squeeze of lemon or alongside your favorite chutney or yogurt dip. Enjoy the fragrant spices and satisfying crunch with every bite.