Season flank steak thoroughly with salt and black pepper. In a large Crock Pot combine steak, bell pepper, onions, ginger and garlic. In a mixing bowl or measuring cup combine beef broth, soy sauce, brown sugar, rice wine vinegar and sesame oil. Season to taste with salt and pepper before adding into the Crock Pot.
Cover and cook on high for 3 1/2 hours to 4 hours or on low for 5 to 6 hours, until the steak is totally tender. Remove the garlic and ginger.
Optional step: If you desire the sauce to be a bit thicker, in a small bowl combine cornstarch and 1/4 cup of the cooking liquid to create a slurry. Stir the slurry into the Crock Pot, cover and cook on high until thickened, an additional 10 to 15 minutes.
Serve over rice and garnish with scallion and white sesame seeds.