Begin by cutting the boneless chicken thighs into bite-sized pieces, then place them in a bowl. Sprinkle freshly ground black pepper and a pinch of salt over the chicken, combining it well. Allow the chicken to marinate for at least 15 minutes to infuse the flavors.

In a large skillet, heat 2 tablespoons of vegetable oil over medium-high heat. Once the oil shimmers, carefully add the marinated chicken pieces to the skillet. Stir-fry them for about 5-7 minutes, or until they turn golden brown on the outside but are not fully cooked. Remove the chicken and set it aside.

Using the same skillet, add the sliced onion and green bell pepper. Sauté the vegetables for 2-3 minutes, allowing them to soften and slightly caramelize.

Next, introduce the minced garlic into the skillet with the sautéed vegetables. Cook for about 1 minute, stirring constantly until the garlic is fragrant but not browned.

Return the browned chicken to the skillet. Pour in 1 tablespoon each of soy sauce and oyster sauce, along with 1 teaspoon of sugar. Stir everything together thoroughly.

In a small bowl, mix 1 tablespoon of cornstarch with 2 tablespoons of water to create a slurry. Pour this slurry into the skillet, stirring quickly until the sauce begins to thicken.

Adjust the seasoning with additional salt if necessary, then reduce the heat to medium. Let the mixture simmer for about 5 minutes.

Finally, garnish the Black Pepper Chicken with finely chopped green onions before serving. Serve the dish hot over steamed jasmine rice or noodles.