Begin by cutting the chicken thighs into bite-sized pieces and placing them in a mixing bowl. Generously sprinkle freshly ground black pepper and a pinch of salt over the chicken. Allow to marinate for at least 15 minutes.

Heat 2 tablespoons of vegetable oil over medium-high heat. Add the marinated chicken in a single layer, cooking for 5-7 minutes until golden brown. Transfer the chicken to a plate and set aside.

In the same skillet, add more oil if necessary, then toss in sliced onion and green bell pepper. Sauté for 2-3 minutes until softened and aromatic.

Once tender, add minced garlic to the skillet and sauté for about 1 minute until fragrant.

Return chicken to the skillet along with juices. Pour in soy sauce, oyster sauce, and sprinkle sugar. Stir well and cook for an additional 2-3 minutes.

In a small bowl, mix 1 tablespoon cornstarch with 2 tablespoons water until smooth. Gradually pour this into the skillet, stirring constantly, and cook for another 2 minutes until the sauce thickens.

Taste and adjust seasoning with salt if needed. Cook for 5 minutes over medium heat until chicken reaches 165°F and is tender.

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