Make the sauce: In a small bowl, whisk together 3/4 tsp black pepper, light soy sauce, dark soy sauce, rice vinegar, brown sugar, toasted sesame oil, 1 tsp cornstarch, and water/broth. Whisk until completely smooth with no cornstarch lumps. Set aside (stir again right before pouring—cornstarch settles quickly).
Coat the chicken: Add chicken cubes to a bowl and season with a pinch of salt, 1/4 tsp freshly ground black pepper, and 2 tbsp cornstarch. Toss until every piece is lightly coated. Let rest 5–10 minutes while you cut onions and bell pepper.
Sear the chicken (work in batches): Heat 1 tbsp peanut oil in a wok or large skillet over medium-high heat. Add chicken in a single layer. Let it cook undisturbed for about 2 minutes to brown, then flip and cook until lightly golden and nearly cooked through (another 2–3 minutes). Transfer to a plate. Repeat with remaining chicken if needed—avoid overcrowding so it browns instead of steaming.
Stir-fry aromatics: Add the remaining 1 tbsp oil to the pan. Add minced garlic, minced ginger, and chopped chili (if using). Stir constantly for 20–30 seconds until fragrant (do not let garlic brown).
Cook vegetables crisp-tender: Add onions and bell pepper. Stir-fry on high heat for 2–3 minutes until the edges soften slightly but the vegetables still have bite. This keeps the final dish crisp and prevents watery sauce.
Thicken the sauce: Reduce heat slightly. Stir the sauce again, then pour it into the pan. Let it bubble for 20–30 seconds, stirring, until it turns glossy and begins to coat the vegetables.
Return chicken and finish: Add the chicken back to the pan and toss quickly to coat. Cook 1–2 minutes until chicken is fully cooked through and sauce clings to every piece. Turn off heat and add an extra pinch of freshly cracked black pepper for aroma if desired.
Serve: Serve immediately over steamed rice or noodles. Best enjoyed fresh while the vegetables are crisp and the sauce is glossy.