Heat 2–3 inches of oil in a wok or deep pan to 350°F (175°C). Fry the chicken in small batches for 4–5 minutes per batch, turning once, until deep golden and crunchy. Don't crowd the pan — crowding drops the oil temperature and makes the coating soggy. Drain on a wire rack (not paper towels — a rack keeps the bottoms crisp). For extra crunch, do a quick second fry of 1 minute at 375°F (190°C).

Pour off the frying oil and wipe the wok clean. Heat 2 tablespoons of fresh oil over high heat until shimmering. Add the garlic, ginger, and green chilies and stir-fry for 30–40 seconds until fragrant but not browned. Add the spring onion whites and capsicum squares; toss for 1–2 minutes so the capsicum stays bright and slightly crunchy.

Stir in the soy sauce, oyster sauce, vinegar, sugar, and black pepper. If making the gravy version, pour in the chicken stock and bring to a boil. For the dry version, skip the stock and add just 2–3 tablespoons of water. Give the cornstarch slurry a quick re-stir (it settles fast), then drizzle it in while stirring constantly. Within 30–60 seconds the sauce will turn thick and glossy — it should coat a spoon. Taste and adjust: more soy for saltiness, a pinch more sugar if too sharp.

Add the fried chicken pieces to the wok and toss vigorously for 1–2 minutes so every piece gets lacquered in sauce. Work fast — the longer the chicken sits in the sauce, the softer the coating gets. This is the secret to chicken manchurian that stays crispy.

Transfer to a serving bowl immediately. Shower with spring onion greens and sesame seeds if using. Serve right away while the coating is still crunchy — chicken manchurian waits for no one.