Saute the vegetables in oil until onions are clear, remove from pan and set aside. Cook chicken until nearly done, add garlic and cook another minute or two.
Add soy sauce and oyster sauce to chicken. Add a few tablespoons of chicken stock to cornstarch and the rest to the chicken. Shake up the cornstarch mixture in a water-tight container, then slowly whisk in to chicken. Adjust sauce to desired thickness, then add vegetables and simmer a few more minutes.