Cut pork into 4x1/4-in. strips; place in a large resealable plastic bag. Add garlic and 2 tablespoons soy sauce; seal bag and turn to coat. Refrigerate for 2-4 hours.
In a small bowl, combine the cornstarch, ginger, broth and remaining soy sauce until smooth; set aside.
Heat oil in a large skillet or wok over high heat; stir-fry pork until no longer pink. Remove and keep warm. Stir-fry carrots and celery for 3-4 minutes. Add the onion, cabbage and spinach; stir-fry 2-3 minutes longer.
Stir cornstarch mixture; stir into vegetables. Return pork to the pan. Bring to a boil; cook and stir for 3-4 minutes or until thickened. Serve with rice if desired.