Yes, you can use the green part as well. It cooks much quicker then the stem piece. Chinese restaurants tend to use it in wonton soup sometimes (both the green and white parts, if you have ever see the green in soup).
Jessica
(HW: 305) (SW: 271.9) (33.1 lbs lost prior to surgery) (MsJexi on MFP)
I actually made some last night! In a pan with a little sesame oil (VERY LITTLE) I sauteed the bok choy with garlic powder and pepper until the green leafs looked almost like cooked spinach... it was sooo yummy!