Cut the chicken into small, uniform pieces, approximately 1 inch by ½ inch. Place them in a mixing bowl.
Add the soy sauce, chili sauce, black pepper, chili powder (if using), salt, cornstarch, and all-purpose flour. Mix thoroughly.
Incorporate the egg white or a few spoonfuls of water gradually, just enough to bind the dry ingredients with the chicken. Mix until evenly coated. Set aside to marinate while preparing the remaining components. Refrigeration overnight is optional.
Prepare the vegetables by cubing the onion and bell pepper. Separate the onion layers. Slit the green chilies lengthwise and set aside.
In a small bowl, combine soy sauce, chili sauce, ketchup, red chili powder (if using), vinegar, sugar, and water to form the sauce base. Stir until smooth and set aside.
Heat approximately one inch of oil in a frying pan over medium heat. To check readiness, drop a small amount of batter into the oil—it should sizzle and rise steadily without excessive browning.
Gently slide in the chicken pieces one by one, taking care not to overcrowd the pan.
Fry over medium-high heat, turning occasionally, until the chicken becomes golden, crisp, and cooked through. Avoid overcooking to retain juiciness.
Once cooked, remove the chicken and let excess oil drain on a paper towel or wire rack. For added crispness, you may re-fry briefly over medium heat.
In a separate wok or pan, heat the oil on high flame. Add the chopped garlic and sauté for about one minute until fragrant.
Add the onions, capsicum, green chilies, and white parts of the spring onions. Stir-fry for about a minute until slightly softened but still crunchy.
Pour the prepared sauce mixture into the wok. Let it bubble and slightly thicken. Taste and adjust seasoning or spice levels as desired.
Toss in the fried chicken and stir well to coat every piece thoroughly with the sauce. Continue to stir-fry over high heat for one minute.
Finish by sprinkling a pinch of black pepper and garnishing with chopped spring onion greens. Serve hot as a standalone appetizer or as an accompaniment to fried rice or stir-fried noodles.