Mix the chicken pieces with egg, cornstarch, black pepper powder, and salt. Set aside.

Heat oil in a deep pan and fry the chicken pieces until golden brown. Remove and set aside.

In the same oil, sauté the onion, garlic, and green bell pepper until softened.

Add sauce (if using), mix well, then add the fried chicken pieces. Cook on low heat until the chicken is cooked through.

Sprinkle with black pepper powder, remove from heat, and serve.