Heat oil in a large skillet over medium heat. Season chicken with salt and onion powder, and place in skillet. Cook for about 5 to 7 minutes, then add the garlic, 4 tablespoons soy sauce, and half of the 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 the chicken mixture. Stir in 4 tablespoons soy sauce, bell pepper, tomatoes, and remaining onion. Simmer until gravy has reached desired consistency.