Place chicken strips, cornstarch, soy sauce, ginger, and garlic powder in a large resealable
plastic bag. Shake bag to fully coat chicken with marinade. Squeeze out excess air and seal bag.
Place bag in refrigerator for 1 to 1 1/2 hours to marinate the chicken strips.
Cook spaghetti according to package instructions. Drain. Set aside.
Place 3 tablespoons vegetable oil in a large skillet. Place over medium heat.
Remove chicken from marinade and shake off excess.
When oil is hot and shimmering, place chicken in skillet. Cook, stirring constantly, in hot oil until chicken is no longer pink in the center, about 5 minutes.
Add onion, celery, carrots and broccoli. Add 1 tablespoon vegetable oil, if needed.
Cover and cook until vegetables are tender, stirring occasionally, about 5 minutes.
Add spaghetti to chicken and vegetable mixture. Stir to mix. Cover and simmer for 5 minutes more.
Serve hot.