In a bowl, mix the soy sauce, oyster sauce, corn-starch, and half of the minced garlic. Add the sliced chicken to this mixture, ensuring it's well coated. Let it marinate for 10 minutes.

In a wok or a large pan, heat 1 tablespoon of vegetable oil over medium-high heat. Add the marinated chicken and stir-fry until it's cooked through and no longer pink. Remove the chicken from the pan and set it aside.

In the same pan, add the remaining tablespoon of vegetable oil. Add the remaining minced garlic and sauté for a minute until fragrant.

Add the sliced red onions and bell peppers to the pan. Stir-fry for about 3-4 minutes until they start to soften but still have a bit of crunch.

Return the cooked chicken to the pan with the vegetables. Add the freshly ground black pepper and a pinch of salt. Stir-fry everything together for another 2-3 minutes to combine the flavours.

Taste and adjust the seasoning if needed. If you prefer a spicier dish, you can add more black pepper at this point.

Once everything is well combined and heated through, remove from heat.