Slice the boneless skinless chicken breasts into thin strips.
Place the sliced chicken in a mixing bowl.
Add the cornstarch, salt, and black pepper to the chicken.
Toss until the chicken is evenly coated with the cornstarch mixture.
Heat 1 tablespoon of vegetable oil in a large skillet over medium heat.
Add the coated chicken to the skillet in an even layer.
Cook for 5 to 7 minutes, stirring occasionally, until the chicken is browned and fully cooked.
Remove the cooked chicken from the skillet and set aside.
Add the remaining 1 tablespoon of vegetable oil to the same skillet.
Add the chopped onion and sliced mushrooms.
Sauté for about 5 minutes, or until the onion is softened and the mushrooms are golden.
Add the minced garlic to the skillet.
Stir in the soy sauce, oyster sauce, sesame oil, and water.
Mix thoroughly to combine the sauce ingredients with the onion and mushrooms.
Bring the sauce to a gentle simmer.
Continue simmering for a few minutes, stirring occasionally, until the sauce slightly thickens.
Return the cooked chicken to the skillet.
Toss the chicken, mushrooms, and onion together until evenly coated in the sauce.
Season with freshly ground black pepper to taste.
Cook for an additional 1 to 2 minutes, or until the chicken is heated through and the sauce is evenly distributed.
Remove the skillet from the heat.
Garnish with sliced green onions, if desired.
Serve hot with steamed rice or noodles.