Marinate chicken with egg, soy sauce, garlic-ginger paste, pepper, salt, cornflour, and flour; mix well and rest 10-15 minutes.
Heat oil in a pan/wok; fry chicken pieces in batches until golden and crispy (about 4-5 minutes); drain on paper towels.
In the same pan, heat 2 tbsp oil; sauté minced garlic, ginger, and green chilies for 30 seconds.
Add sliced onion and bell pepper; stir-fry on high heat for 1-2 minutes until slightly soft but crunchy.
Pour in soy sauce, chilli sauce, ketchup, vinegar, and sugar; mix and cook 1 minute until sauce thickens slightly.
Add fried chicken pieces; toss everything on high heat for 1-2 minutes to coat evenly.
Garnish with chopped spring onions and sesame seeds; serve hot.