Make the black pepper sauce by whisking together the crushed black pepper, rice vinegar, soy sauce, brown sugar, toasted sesame oil, cornstarch, and water in a bowl until there are no lumps. Coat chicken with 1 tablespoon of the sauce, 2 tablespoons of cornstarch, and salt. Rest it aside while you prepare the onion, ginger, garlic, and pepper.

Heat a large frying pan until it's hot. Add one tablespoon of oil and tilt the pan to spread oil all over. Add chicken in a single layer and cook over medium flame until golden from the bottom. Then flip the chicken cubes over and cook until the chicken begins to brown and is almost cooked through. Then transfer the chicken to a plate. Make sure not to overcrowd the pan; cook the chicken in batches if needed.

Heat 1 tablespoon oil in the same pan. Add ginger, garlic, and chopped chilies; Saute for 30 seconds. Immediately add onion and bell peppers. Stir fry over a high flame until the vegetables are crisp and tender. Reduce the flame to the lowest. Whisk the sauce to recombine and pour the sauce. Let it simmer for 30 seconds. Add the fried chicken and toss well until warmed through.