Place the thinly sliced beef chuck roast into the slow cooker.
In a medium bowl, whisk together the soy sauce, brown sugar, rice vinegar, water, minced garlic, grated ginger, sesame oil, crushed red pepper flakes, and black pepper until the sugar dissolves.
Pour the sauce mixture over the beef in the slow cooker, stirring to coat all the beef evenly.
Cover and cook on low for 6 hours, or until the beef is tender and easily shredded.
Once cooked, remove about half of the beef and shred it using two forks. Return the shredded beef to the slow cooker and stir to combine.
In a small bowl, mix the cornstarch and water to create a slurry.
Pour the cornstarch slurry into the slow cooker and stir well to thicken the sauce.
Turn the slow cooker to high and cook for an additional 15 minutes until the sauce thickens.
Serve the Korean beef over steamed rice or noodles, garnished with sliced green onions and toasted sesame seeds.