In a large bowl combine the chicken pieces, 2 tbsp light soy sauce, 3 tbsp cornstarch, 1 beaten egg, 1 tsp salt, and 1/2 tsp black pepper. Mix thoroughly so each piece is coated. Let sit for 15 minutes (or refrigerate up to 1 hour).

Prepare for frying

Heat a large heavy skillet or wok over medium-high heat. Add 4 tablespoons vegetable oil and heat until shimmering (about 350°F/175°C if using a thermometer).

Shallow-fry the chicken

Working in a single layer (fry in two batches if necessary), add the marinated chicken pieces to the hot oil. Fry, turning occasionally, until golden brown and cooked through, about 4–6 minutes per batch depending on size. Remove with a slotted spoon to a wire rack or paper towel-lined plate to drain. Add remaining 2 tbsp oil back to the pan if needed for the sauce step.

Sauce & Stir-fry

Make the sauce

In a small bowl whisk together 3 tbsp ketchup, 1 tbsp dark soy sauce, 1 tbsp light soy sauce, 1 tbsp rice vinegar, 1 tsp sugar, and 1/4 cup water until smooth. In a separate small cup dissolve 1 tsp cornstarch in 1 tbsp water to make a slurry. Set both aside.

Sauté aromatics and vegetables

Using the same skillet or wok over high heat, add the remaining oil if necessary and let it get very hot. Add the minced garlic (4 cloves), minced ginger (1 tbsp), sliced fresh red chilies (2) and white parts of the sliced green onions. Stir-fry for 20–30 seconds until fragrant but not burned. Add the onion wedges and bell pepper pieces. Stir-fry 1.5–2 minutes until vegetables are bright and slightly tender-crisp.

Combine sauce and thicken

Pour the prepared sauce into the wok. Bring to a quick simmer, scraping up any browned bits. Stir in the cornstarch slurry, stirring constantly, until the sauce thickens and becomes glossy (about 30–45 seconds). Remove from heat and stir in 1/2 tsp sesame oil.

Finish with chicken

Return the fried chicken pieces to the wok and toss to coat thoroughly in the sauce and vegetables. Sprinkle the dried red chili flakes (1 tsp) and the green parts of the sliced green onions. Toss for another 30 seconds over medium-high heat so the chicken is heated through and coated evenly.

To Serve

Transfer the chilli chicken to a serving platter. Garnish with chopped cilantro if using. Serve hot with steamed rice or noodles.