Cut chicken into 1 cm strips; place in a resealable plastic bag. Add cornstarch and toss to coat. Combine soy sauce, ginger and garlic powder; add to bag and shake well. Refrigerate for 30 minutes. In a large skillet or wok, heat 2 Tablespoon of oil; stir-fry chicken until no longer pink, about 3-5 minutes. Remove and keep warm. Add remaining oil; stir-fry broccoli, celery, carrots and onion for 4-5 minutes or until crisp-tender. Add water and bouillon. Return chicken to pan. Cook and stir until thickened and bubbly.