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 4 tablespoons soy sauce, garlic and half of the sliced onion. Cook until chicken is no longer pink. Dissolve cornstarch in water in a small bowl, and stir into the chicken mixture. Stir in 4 tablespoons soy sauce, bell pepper, tomatoes, and remaining onion. Simmer until gravy has reached desired consistency. Makes 4 servings.