Place beef chuck roast into the slow cooker.
In a bowl, whisk together soy sauce, brown sugar, garlic, ginger, sesame oil, rice vinegar, and red pepper flakes.
Pour the sauce over the beef and cook on low for 7–8 hours until tender.
Shred the beef using two forks directly in the slow cooker.
Mix cornstarch with water and stir into the sauce to thicken slightly.
Cook noodles according to package instructions and drain well.
Toss noodles with the shredded Korean beef and sauce until fully coated.
Garnish with sesame seeds and fresh parsley or green onions before serving.
