Dredge the sliced flank steak evenly in cornstarch, ensuring all pieces are well coated.
Arrange the coated steak slices in the slow cooker.
In a separate bowl, combine the sesame oil, garlic, ginger, soy sauce, brown sugar, and water. Stir until the mixture is uniform.
Pour the prepared sauce over the steak in the slow cooker, ensuring the meat is fully immersed.
Cover and cook on the HIGH setting for 2-3 hours or on the LOW setting for 4-5 hours, allowing the flavors to meld as the meat becomes tender.
Mix in the sliced green onions just before serving.
Serve this flavorful dish over steamed rice or with your choice of accompaniments.