Slice the chicken breasts into thin strips. Slice the red bell pepper, green bell pepper, and onion. Mince the garlic cloves.
Heat the vegetable oil in a large pan over medium-high heat.
Add the sliced chicken to the pan. Cook for 5–6 minutes while stirring until the chicken turns golden and is fully cooked.
Add the minced garlic and stir for about 30 seconds until fragrant.
Add the sliced onion, red bell pepper, and green bell pepper. Stir-fry for 3–4 minutes until the vegetables become slightly soft but still crisp.
Pour in the soy sauce, chili sauce, and oyster sauce. Mix everything well so the chicken and vegetables are fully coated.
Add the black pepper, chili flakes, and salt to taste. Stir well and cook for another 2 minutes.
Turn off the heat and serve the spicy chicken stir fry hot with rice or noodles.