In a slow cooker, combine the soy sauce, brown sugar, rice vinegar, sesame oil, garlic, ginger, and red pepper flakes. Stir until well mixed.
Add the sliced flank steak to the slow cooker, ensuring the beef is well coated with the sauce.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the beef is tender.
In a small bowl, mix the cornstarch and water to create a slurry. About 30 minutes before serving, stir the slurry into the slow cooker to thicken the sauce.
Once cooked, serve the Korean beef over cooked rice and garnish with sliced green onions and sesame seeds.