Place the chicken in a resealable plastic bag. Add the cornstarch and toss to coat
In a bowl combine the soy sauce, ginger and garlic powder. Add the mixture to the bag and shake well. Refrigerate for 30 minutes
Meanwhile cook the rice as per packet instructions. In a large pan, heat 2 tablespoons of oil. Remove the chicken from the bag but leave the marinade in it. Add the chicken and stir-fry until no longer pink - approx 3-5 minutes
Remove the chicken and keep warm. Add the remaining oil and stir-fry the broccoli, celery, carrots and onion for 4-5 minutes or until crisp but tender
Return the chicken to the pan and add the marinade from the bag. Cook until the chicken is warmed through. Keep stirring whilst cooking to prevent the food from sticking