Cut the chicken breast halves into ½-inch strips. In a bowl, combine cornstarch, soy sauce, and minced garlic to make a marinade. Toss the chicken strips in the marinade until coated and let marinate for at least 30 minutes.

Heat 2 tablespoons of oil in a large skillet or wok over medium-high heat. Add the marinated chicken and stir-fry until no longer pink, then remove and set aside. Add the remaining tablespoon of oil to the pan and stir-fry the broccoli, celery, carrots, and onion until crisp-tender, about 3-4 minutes.

Pour in the chicken stock and return the cooked chicken to the pan. Stir until the mixture is bubbly and thickened, coating all ingredients in a glossy sauce.