Heat oil in a large skillet over medium heat. Season chicken with salt and onion powder and place in skillet. Cook for 5 to 7 minutes.

Add garlic, 4 tablespoons soy sauce, and half of sliced onion. Cook until chicken is no longer pink and the juices run clear.

Dissolve cornstarch in water in a small bowl, and blend into chicken mixture. Stir in remaining 4 tablespoons soy sauce, bell pepper, tomatoes, and remaining onion. Simmer until gravy has reached desired consistency.