Combine chicken pieces with freshly ground black pepper and a pinch of salt in a bowl. Let marinate for at least 15 minutes.

Heat vegetable oil in a large skillet over medium-high heat. Add chicken in a single layer and sear until golden brown on the outside but slightly undercooked inside. Do not crowd the pan. Work in batches if needed. Remove and set aside.

In the same skillet, add sliced onion and green bell pepper. Saute for 2 to 3 minutes, stirring frequently, until edges soften and begin to color.

Lower heat to medium, add minced garlic, and cook for about 1 minute until fragrant. Do not let it brown.

Return chicken to the skillet. Add soy sauce, oyster sauce, and sugar. Stir well to coat everything evenly.

Mix cornstarch with water in a small bowl until smooth. Pour into the skillet and stir continuously until the sauce thickens to a glossy consistency.

Taste and adjust salt if needed. Continue cooking for about 5 more minutes until the chicken is fully cooked through.

Garnish with chopped green onions and serve immediately over steamed jasmine rice.