Preheat your slow cooker on the low setting.
In a large bowl, whisk together the soy sauce, brown sugar, rice vinegar, sesame oil, garlic, ginger, black pepper, and red pepper flakes until well combined.
Place the short ribs in the slow cooker and pour the marinade over them, ensuring they are well coated.
Cover the slow cooker and cook on low for 6-8 hours, or until the meat is tender and easily pulls away from the bone.
Once cooked, carefully remove the short ribs and place them on a serving platter.
Strain the cooking liquid through a fine mesh sieve into a saucepan and bring to a simmer over medium heat. Cook for about 10 minutes, or until slightly thickened.
Drizzle the thickened sauce over the short ribs and garnish with chopped green onions and sesame seeds before serving.