Slice meat into thin strips and toss in a Ziploc bag with cornstarch. Add all remaining ingredients to your crockpot and mix well. Put your meat on top, and toss gingerly to coat.
Cover and cook on low for 3-4 hours. Flank steak is thin and has very little fat, and will cook quickly. There isn't a lot of liquid in this dish, so if your crock tends to cook hot, please check it after 3 hours. The meat is done when it is no longer pink and has reached desired tenderness.
Serve over steamed brown rice, and garnish with freshly sliced green onion.
This is even better the next day. Next time try it without the peanut butter.