Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the chicken pieces to the skillet and season with salt. Cook until browned and cooked through, about 5-7 minutes, stirring occasionally.
Once the chicken is cooked, add the slices of onion, bell peppers, and minced garlic to the skillet. Stir-fry for another 3-4 minutes until the vegetables are tender.
In a small bowl, combine black pepper, soy sauce, oyster sauce (if using), and sugar. Pour this mixture over the cooked chicken and vegetables in the skillet.
Stir well to coat everything evenly and cook for an additional 2-3 minutes to allow the flavors to meld.
Taste and adjust seasoning with more salt or additional black pepper if desired.
Remove from heat, garnish with chopped green onions, and serve over cooked rice.
For a spicier kick, consider adding some red pepper flakes or sliced chili peppers to the dish.
You can substitute chicken thighs with chicken breast if preferred, but thighs will yield a juicier end result.
This dish pairs well with steamed vegetables or a side salad for a complete meal.