Mix all the above ingredients together and keep covered for about 1 hr. Deep fry chicken pieces in medium hot oil and set aside.
Heat 2 tbsp oil in a small wok and fry garlic until light golden. Then add chopped onion, capsicum along with fried chicken pieces and fry for 1 min on high heat.
Lower the heat and add rest of the ingredients – red chili powder, tomato sauce, tomato puree, soy sauce, sugar, 2 cups water, and vinegar. Add salt only if needed.
Bring to a boil and slowly add cornflour mixture little by little according to the thickness you need.
Cook for a couple of minutes and finally add chopped green spring onion.