01 - In a large bowl, combine the chicken pieces with soy sauce, vinegar, ginger-garlic paste, cornstarch, all-purpose flour, black pepper, salt, and egg. Toss thoroughly until every piece is evenly coated. Allow the chicken to marinate for at least 15 minutes at room temperature.
02 - Heat about 1 inch of oil in a deep pan or wok over medium-high heat until it reaches 350°F. Fry the marinated chicken pieces in small batches for 3 to 5 minutes per batch, turning occasionally, until golden brown and crispy on all sides. Transfer each batch to a paper towel-lined plate to drain excess oil.
03 - In a large wok or skillet, heat 3 tablespoons of vegetable oil over medium-high heat. Add the chopped garlic, green chilies, and diced onion. Sauté for 1 to 2 minutes, stirring frequently, until the aromatics are fragrant and the onions begin to soften.
04 - Add the diced green and red bell peppers along with the white parts of the spring onions. Stir-fry for another 2 minutes, keeping the vegetables crisp-tender with a slight char on the edges.
05 - In a small bowl, whisk together the soy sauce, chili sauce, tomato ketchup, oyster sauce, sugar, and water until well combined and the sugar has dissolved.
06 - Pour the sauce mixture into the wok with the vegetables and stir well to combine. Bring the sauce to a gentle simmer, allowing it to bubble and reduce slightly for about 1 minute.
07 - Add the fried chicken pieces to the wok and toss vigorously to coat every piece evenly in the sauce. Continue tossing for 1 to 2 minutes over medium-high heat.
08 - Pour the cornstarch slurry into the wok while stirring continuously. The sauce will thicken quickly and form a glossy glaze that clings to the chicken and vegetables. Stir for 30 seconds to 1 minute until the coating is uniform.
09 - Remove from heat and garnish generously with the reserved spring onion greens. Serve immediately while hot, accompanied by steamed jasmine rice or fried rice.