Place the sliced beef into the slow cooker basin.
In a medium bowl, whisk together soy sauce, brown sugar, garlic, ginger, sesame oil, rice vinegar, red pepper flakes, and beef broth until the sugar is dissolved.
Pour the sauce mixture over the beef and stir to coat evenly.
Cover and cook on Low for 6 to 8 hours or High for 3 to 4 hours until the beef is tender.
Approximately 10 minutes before serving, cook the noodles in boiling water according to package directions, then drain.
Add the cooked noodles to the slow cooker and toss with the beef and sauce to combine.
Serve hot, garnished with sliced green onions and sesame seeds.