1. Heat sufficient oil in a kadai.
2. Combine refined flour, ¼ cup cornstarch, ginger-garlic paste and salt in a bowl. add some water and mix to make a smooth and semi-thick batter.
3. Add button mushrooms and mix till well coated.
4. Put the button mushrooms in hot oil and deep-fry till crisp. drain on absorbent paper.
5. Heat 1 tablespoon oil in a non-stick pan. add garlic and ginger and sauté for 30 seconds.
6. Add green chillies and sauté for 30 seconds. add spring onions and sauté for 1 minute.
7. Add ¼ cup water and mix. add tomato ketchup and soy sauce and mix well.
8. Mix together remaining cornstarch with 1-2 tablespoons water to make a smooth slurry. add to pan, mix and cook till the mixture thickens.
9. Add fried mushrooms and vinegar and mix. add salt and 1 tablespoon spring onion greens and mix well.
10. Serve hot garnished with spring onion greens.