Arrange the short ribs in the bottom of a slow cooker or pan, then sprinkle on the thinly sliced onion.
Mix together the soy sauce, beef broth, rice vinegar, brown sugar, black pepper, sesame oil, garlic, ginger, and red pepper flakes. Pour over the short ribs.
Cover and cook on low heat for 6-8 hours or on high heat for 3-4 hours. You can also do this in the oven for 4 1/2 hours at 350. When almost ready to serve, whisk the corn starch and cold water together and pour into the slow cooker, stirring to combine with the sauce and juices from the meat.
Cover and cook for another 20 minutes until the liquid has thickened slightly. Serve with rice.