In a mixing bowl, combine the sliced chicken thighs, soy sauce, and corn starch. Mix well to coat the chicken evenly and let it marinate for about 10 minutes.
Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the marinated chicken to the skillet in a single layer. Cook for 4-5 minutes without stirring, allowing the chicken to sear.
Stir the chicken and cook for another 4-5 minutes until it's cooked through and no longer pink in the center. Remove the chicken from the skillet and set aside.
In the same skillet, add the sliced onion and bell pepper. Stir-fry for about 3 minutes until they are slightly softened.
Add the minced garlic and ginger to the skillet and cook for an additional 1 minute until fragrant.
Return the chicken to the skillet and sprinkle in the black pepper and sugar. Stir well to combine.
Add the water and cook for another 2-3 minutes, allowing the sauce to thicken slightly and coat the chicken.
Garnish with sliced green onion and serve hot with rice or vegetables.