Prepare the flank steak by cutting thin slices against the grain. The slices should be about ⅛ to ¼ inch thick.
In a bowl or Ziplock bag, add the slices of flank steak, cornstarch, salt and pepper.
Gently mix the meat in the bowl or bag to cover the flank steak in the cornstarch and seasonings. Let the meat rest for 5 minutes.
Place the flank steak on the bottom of the slow cooker.
Add the sesame oil, ginger, garlic, soy sauce, brown sugar and water on top of the sliced flank steak.
Give it a good mix and then put the cover on your slow cooker.
Cook on low for 3 hours.
After 3 hours, remove the slow cooker lid and stir in the sliced bell peppers and green onions.
Cook for 1 additional hour, the meat should be tender, the sauce dark brown and thick.
Before serving, top with diced green onions and sesame seeds if you wish.