In a bowl, combine soy sauce, oyster sauce, corn-starch, and half of the minced garlic. Coat the sliced chicken with this mixture and marinate for 10 minutes.
Heat 1 tablespoon of vegetable oil in a wok or large pan over medium-high heat. Stir-fry the marinated chicken until fully cooked. Set aside.
In the same pan, add another tablespoon of vegetable oil and sauté the remaining minced garlic until fragrant.
Toss in sliced red onions and bell peppers, stir-frying for 3-4 minutes until slightly softened.
Return the chicken to the pan, adding black pepper and a pinch of salt. Stir-fry for an additional 2-3 minutes.
Adjust the seasoning to taste, adding more black pepper for extra spice.
Remove from heat once everything is heated through and well combined.
Serve the Chicken Black Pepper Stir-Fry with steamed rice or noodles. Top with chopped green onions for freshness.