Put the chicken pieces in a bowl. Add the freshly ground black pepper and a pinch of salt. Toss to coat. Let sit for at least 15 minutes.

Heat the vegetable oil in a large skillet over medium-high heat. Add the marinated chicken in a single layer. Stir-fry until browned but not fully cooked, about 4 to 5 minutes. Remove the chicken to a plate.

In the same skillet, add the sliced onion and green bell pepper. Sauté for 2 to 3 minutes until they begin to soften.

Add the minced garlic and cook for 30 to 60 seconds until fragrant.

Return the chicken to the skillet. Pour in the soy sauce and oyster sauce, then sprinkle the sugar. Stir to coat all pieces.

In a small bowl, whisk the cornstarch and water into a smooth slurry. Pour the slurry into the skillet and stir continuously until the sauce thickens and becomes glossy.

Taste and adjust salt if needed. Continue cooking until the chicken is fully cooked and tender, about 3 to 5 more minutes. The internal temperature should reach 165°F (74°C).

Remove from heat and garnish with chopped green onions. Serve immediately over rice or noodles.