Separate the onion roundels into rings and keep the center part aside. Use only the outer big rings. Sieve together refined flour, cornstarch, salt, white pepper powder and baking powder. Add half cup of oil and sufficient water to make a thick batter. Whip this batter with whisk or hand continuously for a long time - say five to ten minutes. Heat sufficient oil in a kadai. Dip onion rings in the batter and deep fry till crisp. Remove and drain on an absorbent paper. Serve hot.