Pat the chicken pieces dry with paper towels. In a large bowl combine cornstarch, all-purpose flour, salt, and black pepper. Beat the egg lightly and add to the bowl along with the ginger-garlic paste. Mix into a thick batter; if too dry, add 1–2 tablespoons water to reach a coating consistency. Toss the chicken pieces so each piece is evenly coated with the batter.

Fry the chicken

Heat oil in a deep skillet or heavy pot to 350°F (175°C) or until a small bit of batter sizzles and rises. Fry the coated chicken in batches (do not overcrowd) for 4–6 minutes per batch until golden brown and cooked through. Use a slotted spoon to transfer chicken to a paper-towel-lined tray to drain. Keep warm in a low oven (200°F / 95°C) while you make the sauce.

For the sauce

Aromatics

Discard excess frying oil leaving about 2 tablespoons in the pan or use fresh 2 tablespoons vegetable oil in a large skillet or wok over medium-high heat. Add minced garlic, minced ginger, and sliced green chilies. Stir-fry for 20–30 seconds until fragrant but not browned.

Vegetables

Add the diced onion and bell pepper to the pan. Stir-fry for 2–3 minutes until they begin to soften but still retain some crispness.

Make the sauce

Reduce heat to medium. Add tomato ketchup, soy sauce, rice vinegar, chili sauce, and brown sugar. Stir to combine. Pour in the 1/2 cup water or chicken stock and bring the mixture to a gentle simmer.

Thicken

Stir the cornstarch slurry again (1 tablespoon cornstarch mixed with 1 tablespoon cold water) and slowly drizzle into the simmering sauce while stirring. Continue cooking 1–2 minutes until the sauce becomes glossy and thick enough to coat the back of a spoon. Taste and adjust salt and pepper; add sesame oil if using.

Combine chicken and sauce

Add the fried chicken pieces and sliced scallion whites/greens (reserve some scallion greens for garnish) to the pan. Toss well to coat each piece in the sauce and heat through for 1–2 minutes so the chicken re-warms and absorbs the sauce.

To finish

Transfer Chicken Manchurian to a serving platter. Garnish with remaining sliced scallion greens and toasted sesame seeds, and serve immediately with steamed rice or fried rice and lime wedges on the side.