Shake flank steak strips and cornstarch in a ziploc bag to coat.
In the slow cooker, mix sesame oil, garlic, soy sauce, beef stock, brown sugar, onion, and red pepper flakes until combined. Stir in flank steak strips until coated in sauce.
Cover and cook on low for 4 to 5 hours, or until steak is cooked through and tender. Serve hot over rice and garnish with sesame seeds and green onions.