Slice the chicken breasts into thin pieces. Slice the mushrooms and onion. Mince the garlic.
In a small bowl, mix the cornstarch with water and keep it aside.
Heat the vegetable oil in a large pan over medium-high heat.
Add the sliced chicken to the pan. Cook for 4–5 minutes until the chicken turns white and lightly golden.
Add the onion, mushrooms, and minced garlic. Stir and cook for 3–4 minutes until the vegetables become soft.
Add the soy sauce, oyster sauce, black pepper, salt, and 2 tablespoons of water. Stir everything well.
Pour the cornstarch mixture into the pan. Stir for 1–2 minutes until the sauce becomes slightly thick.
Turn off the heat and serve the Chicken Mushroom Stir Fry warm.