In a bowl, mix the chicken cubes with cornflour, flour, salt, red chili powder, and black pepper. Add beaten egg and mix well.
Heat oil in a wok and fry the chicken until it turns golden brown. Drain the excess oil and set aside.
In the same wok, add 2 tbsp of oil, garlic, and green chilies. Saute for a minute.
Add onion and capsicum and stir-fry for 2-3 minutes.
Add soy sauce, chili sauce, vinegar, tomato paste, sugar, and salt. Mix well.
Add the fried chicken to the wok and stir-fry for 2-3 minutes.
Garnish with chopped spring onions and serve hot with steamed rice.