In a small bowl, combine cornstarch, broth and soy sauce until smooth; set aside.
Cut off and discard the root end of the bok choy, leaving stalks with leaves. Now cut enough leaves into 1-inch slices to measure 2 cups and cut enough stalks into 1/2-inch pieces to measure 2 cups. Save remaining bok choy for another use.
In a large skillet or wok, heat 1/2 tablespoon of oil over medium-high heat; add pork strips and stir-fry for 3 to 4 minutes or until no longer pink; remove from skillet and keep warm. Add remaining oil to the skillet; stir-fry bok choy stalks for 2 minutes.
Add garlic, gingerroot and bok choy leaves; stir-fry for 2 minutes. Add water and crushed red pepper flakes (if desired); cook for 2 minutes or until bok choy is tender-crisp. Stir reserved cornstarch mixture and add it to the skillet. Bring to a boil; cook and stir for 2 minutes or until thickened.
Add green onions, mango and reserved pork strips; heat through. Stir in sesame oil just before serving.