Marinate the chicken breast - Cut the chicken breast into 2 slices. Cut the chicken breast diagonally into 1/4-inch thick slices.
Marinate the chicken breast with soy sauce, cooking wine, dark soy sauce, sugar, black pepper, and cornstarch. After mixing well, add the sesame oil, mix well and set aside.
Mix the sauce - Mix soy sauce, oyster sauce, water, and black pepper in a medium bowl until well combined. Set aside.
Prepare the vegetables - cut the green bell pepper and onion into 1” cubes.
Stir-fry vegetables - pour oil into a hot pan, add onion and green bell pepper. Stir-fry for 1 minute, transfer to a plate and set aside.
Add the chicken to the same pan and don't stir until the bottom is golden brown. Add the chopped garlic and stir-fry for 1 minute.
Add the sauce and vegetables and stir-fry for 1 minute. Transfer to a plate.
Serve hot with steamed rice.