Slice the chicken breasts into thin strips. Slice the mushrooms and onion, and mince the garlic.
Heat the vegetable oil in a large pan over medium heat.
Add the sliced chicken to the pan. Cook for 5–6 minutes, stirring often, until the chicken turns white and lightly golden.
Add the sliced onion and minced garlic. Stir and cook for 2 minutes until fragrant.
Add the sliced mushrooms and cook for another 3–4 minutes until they become soft.
Pour in the soy sauce and oyster sauce. Stir well to coat the chicken and mushrooms.
Add the black pepper and salt. Mix everything together evenly.
Pour in the 2 tablespoons of water and let the mixture simmer for 2 minutes.
Add the cornstarch mixed with water. Stir well and cook for 1–2 minutes until the sauce becomes slightly thick.