Begin by cutting the boneless chicken thighs into bite-sized pieces. In a bowl, season the chicken with freshly ground black pepper and salt, then mix thoroughly. Allow the chicken to marinate for at least 15 minutes at room temperature.

Heat 2 tablespoons of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated chicken pieces and stir-fry for 5-7 minutes until browned and just cooked through. Remove from skillet.

In the same skillet, add more oil if necessary, then toss in the chopped onion and green bell pepper. Stir-fry for 2-3 minutes until softened but still crunchy.

Add the minced garlic to the skillet and stir continuously for about 1 minute until fragrant and slightly golden.

Return the seared chicken to the skillet and pour in the soy sauce, oyster sauce, and sprinkle in the sugar. Stir well and cook together for about 2 minutes.

In a small bowl, mix cornstarch with 2 tablespoons of water to create a slurry. Pour this into the skillet, stirring continuously until the sauce thickens.

Taste and adjust seasoning with salt if needed. Cook for another 3-5 minutes until flavors deepen.

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