In a small bowl, combine the cornstarch, broth and soy sauce until smooth; set aside.
Cut off and discard root end of boy choy, leaving stalks with leaves. Cut enough leaves into 1-in. slices to measure 2 cups. Cut enough stalks into 1/2-in. pieces to measure 2 cups. Save remaining bok choy for another use.
In a large skillet or wok, stir-fry pork in 1 tablespoon oil for 3-4 minutes or until no longer pink. Remove and keep warm.
Stir-fry bok choy stalks in remaining oil for 2 minutes. Add the garlic, ginger and bok choy leaves; stir-fry for 2 minutes. Add water and pepper flakes if desired; cook 2 minutes longer or until bok choy is crisp-tender.
Stir cornstarch mixture and add to the pan. Bring to a boil; cook and stir for 2 minutes or until thickened. Add the green onions, mango and reserved pork; heat through. Stir in sesame oil. Serve with rice.