In a large mixing bowl, whisk together the soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, gochujang, and black pepper. Stir until the sugar is dissolved and the marinade is well combined.
Add the cubed beef chuck roast to the marinade, ensuring each piece is evenly coated. Cover the bowl and let the beef marinate for at least 30 minutes at room temperature, or for optimal flavor, refrigerate it overnight.
After marinating, transfer the beef along with all the marinade into the slow cooker. Arrange the sliced onion on top of the beef for additional flavor during cooking.
Cover the slow cooker with its lid and set it on low heat. Allow the mixture to cook undisturbed for 6-8 hours, or until the beef is fully tender and can be easily shredded with a fork.
Once the cooking time is complete, use two forks to shred the beef directly in the slow cooker, mixing it with the sauce and caramelized onions to ensure everything is well integrated.
To serve, spoon generous portions of the savory Korean beef over a bed of warm jasmine rice. Garnish each serving with chopped green onions and a sprinkle of sesame seeds for a delightful crunch and burst of flavor.
Notes
Serve in deep bowls to capture the sauce, and consider adding pickled radish on the side.