Place the thinly sliced beef chuck roast in the slow cooker.
In a medium bowl, whisk together the soy sauce, brown sugar, 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 and toss to coat evenly.
Cover and cook on low for 6 hours, or until the beef is tender and easily shredded.
About 15 minutes before serving, remove 1/2 cup of the cooking liquid and place it in a small bowl.
In the small bowl, mix the cornstarch with cold water until smooth to create a slurry.
Stir the cornstarch slurry into the reserved cooking liquid, then pour the mixture back into the slow cooker.
Turn the slow cooker to high and cook uncovered for 10-15 minutes until the sauce thickens.
Stir the beef to coat it well with the thickened sauce.
Serve the Korean beef over steamed rice or noodles, garnished with sliced green onions and toasted sesame seeds.