In a bowl, add all-purpose flour, corn flour, salt, chilli paste and mix well. Add water to make smooth thick paste.
Heat enough oil for deep frying in a heavy bottomed vessel.
Add the washed and drained mushrooms into the batter and ensure it is well coated. Place each mushroom into the hot oil on a medium flame and fry till golden brown. Remove the fried mushroom balls onto absorbent paper and set aside.
Heat 2 tsp oil in a large wok and stir fry chopped garlic and ginger for few seconds.
Add the sliced capsicum, onion and stir fry on high flame for few more seconds.
Reduce flame to medium flame, add sugar, soya sauce, tomato sauce, chilly sauce and vinegar. Mix well and cook for two minutes. Add 3 Tbsp of water and cook for a minute.
Add the golden fried mushroom balls and combine. Toss on high flame for a minute and turn off heat. Garnish with chopped spring onions.
Transfer to a serving bowl and serve hot with any hot and sour sauce.