Pat the short ribs dry and trim excess surface fat if needed.
In a bowl, whisk together the hoisin sauce, soy sauce, vinegar, sesame oil, ginger, and garlic.
Place the short ribs in the slow cooker and pour the sauce over the top. Turn ribs to coat.
Cover and cook on LOW for 8 hours, or until the meat is very tender.
Carefully remove the short ribs and keep warm.
Strain the cooking liquid into a saucepan and skim excess fat from the surface.
In a small bowl, whisk together the cornstarch and water to make a slurry.
Bring the sauce to a simmer and whisk in the slurry. Cook 1 to 2 minutes until thickened.
Return sauce over the ribs or serve on the side.
Garnish with green onions and sesame seeds. Serve hot over rice or mashed potatoes.