Peel off the skin for onion. Slice the onion into rings about 1cm wide, separate the rings.
Soak onion rings in a bowl of ice water for 1 hour, then transfer the onion rings to paper towels to drain.
Preheat cooking oil to 180°C in a wok.
Place crispy fry flour in a bowl. Add in water and cooking oil, stir until well combined.
Coat onion rings with batter evenly. Fry onion rings at medium heat for 3 minutes or until golden brown. Flip the onion rings to get even colour.