In a large bowl, whisk together soy sauce, beef broth, brown sugar, garlic, ginger, sesame oil and red pepper flakes, if using.
Place short ribs into a 6-qt slow cooker. Stir in soy sauce mixture until well combined.
Cover and cook on low heat for 7-8 hours or high heat for 3-4 hours.*
In a small bowl, whisk together cornstarch and 1/4 cup water. Stir in mixture into the slow cooker. Cover and cook on high heat for an additional 30 minutes, or until the sauce has thickened.
Serve immediately, garnished with parsley and sesame seeds, if desired.
Notes
*To test for doneness, pull on a bone as it should slide out freely.